6" Skyjacker lift on a 74 K2500 long bed with driveline vibrations

Looking for those with similar trucks that have 6" suspension lifts and driveline vibrations. I'm already aware of pinion angles and how they should be pointed based on the type of driveshaft. I've been fighting this vibration for awhile now and getting tired of installing and removing/changing pinion shims. I've already installed 1" spacers to lower my transfer case and reduce the angle and my pinion and t-case are currently at the same angle. The driveshaft is a new single cardan shaft with new 1350 spicer u joints. NP203 transfer case. Rhythmic pulsing vibration at 55mph is still there no matter what I do. I've read SOOO many other threads from guys that have had different solutions to this similar problem (balance the shaft, balance the tires, change the angles, replace the pinion bearings, replace the yoke bearings...)

Before I continue chasing ghosts and throwing money I don't have at problems that don't exist, I want to first see if anyone else here has a double cardan rear driveshaft on their lifted long bed truck and if that was the fix. I'm sure there are plenty of 6" lifted squarebodies out there without vibration issues with single cardan shafts- I just want to see if a double cardan is the next most viable option for me at this point. Although I'm not 100% certain it's the guaranteed fix, I'd rather spend $650 on a new CV shaft and be (most likely?) done with it, than spend countless hours and probably more money fixing/replacing (and probably breaking) other things in the process.

*Full disclosure* If you can't already tell, I'm currently suffering from "analysis paralysis" so go easy on me.
Any suggestions are appreciated!

Pics attached for reference.

Have you ensured that it is the driveshaft? Have you pulled the rear shaft and driven 55 on the front only? If the vibration goes away, that would confirm that it is the shaft and not the wheels, or something else? That looks like a bunch of up angle on the rearend; Are you sure that and the TC output shaft are parallel?
Maybe look in here: https://4xshaft.com/blogs/general-tech-info-articles/diagnosing-drive-shaft-vibrations

H2OnSnow said:

Have you ensured that it is the driveshaft? Have you pulled the rear shaft and driven 55 on the front only? If the vibration goes away, that would confirm that it is the shaft and not the wheels, or something else? That looks like a bunch of up angle on the rearend; Are you sure that and the TC output shaft are parallel?
Maybe look in here: https://4xshaft.com/blogs/general-tech-info-articles/diagnosing-drive-shaft-vibrations

Click to expand...

I appreciate the feedback. So I did try to drive on just the front driveshaft to test that theory but my transfer case kept slipping out of 4H - leaving me stuck in neutral. I have a part time kit in it and just couldnt get it to engage to complete that diagnostic. But once I put the rear shaft back on, I was able to engage it in 4H and 4L again. So- short story long-I couldnt determine for sure. The rear and TC are parallel. The original Skyjacker leafs I installed came with a degree shims installed under the leafpacks which had my pinion pointed even higher- directly at the TC. I removed them and check my angles which are perfectly parallel. The truck is parked on an incline in that picture.

My build is NOT 6-inch lift...
The 4L80-E and the 14-bolt rear mod did pose significant spacing issues similar to what you are dealing with though.

Try this:
Spicer Driveline App angle finder:
https://spicerparts.com/calculators/driveline-operating-angle-calculator

And this:

Toolbox TREMEC USA:
https://tremec.com/aftermarket/tremec-toolbox-app/

It took both apps to get my driveline correct because there's a slight bias towards lower angle driveline like in a Camaro or Grand Am in the formulas.

If you use shims and keep the diff angle as low as possible to make both apps agree, then verify the drive shaft and engine RPM limits using the Spicer App, you'll get it.

Try to get as close as you can to the 4500 rpm limits on the Spicer app.

Welded my perches, the axle ended up a little too low (flatter than angled), so I used 2 degree shims to correct them. The shims are not long enough for the extra extended perches with 3 holes, for rear axle spacing adjustments.

When I adjusted the axle to the rear perch holes, and shimmed using the shims into the center hole, had to weld in spacers towards the back of the perches to ADD shim thickness and fill the gap. The steel spacers were not welded to the axle, but rather to the PERCH only. In your case it looks a bit too high, almost straight, under load or hitting bums. If the axle was ever hard pushed into the transmission, then the harmonics could be transmission damage.
Pressing the man shaft forward due to axle hit causes the pump to get crushed at the front of the transmission.

Lot of builders have under estimated the full movement of a solid axle on a high lift.

So under load, or over bumps at speed, your drive line is a straight shot to the axle, if the angle issues to straight or the driveline hits the case. This is BAD.

IF the driveline was too straight, the U-Joint is likely burned up from not properly rotating.

This can happen in just a few hours of driving at the wrong driveline angle.

Buy a new U-Joint just to be safe. Best $35.00 you'll spend if you roasted it in a straight line.

They sell 2 degree wedges and 4 degree wedges and they are not all equal in quality or materials, so choose wisely.
Be prepared to weld in extra material to fill the perches properly after it runs right.

When you try to use the smart phone apps, remember the buttons on the sides of the phone will throw off your angles and screw with your measurements. Take your time. get a friend to assist and never tighten the U-bolts until you are certain they are correct. Most U-Bolts are not good at the same spot repeatedly tightening or worse getting frustrated and over tightening.


If it still will not run right, $250 having the shaft balanced will save your transmission bearings and a lot of other harmonics damages from an improperly angled and balanced shaft.

You'll need at least 2-2-1/2" inches at the slip shaft, if you have one, or the truck will bottom out the shaft into the transmission case over heavy bumps and jumps. It's a balancing act that you must right, or you'll break stuff.

They sell a drive shaft with compression splines and telescopic system that makes up for heavy lift geometry.
$$$$

Call OffRoadDesigns and get the answers if you cannot make it work with my low dough optional fix?

Don't risk your transmission, transfer case bushing and/or rear differential gears over an $800 drive shaft mod.

The T-Case housing is $250 if it cracks, let alone the rebuild. Mine was a bear to get correct.
Looks easy and it is definitely NOT. The synchro's will eat the bronze bushing if you lose a slider spring.

Lifting a truck is very expensive research and testing. Most 6-inch lifted trucks should not be driven over 3500 rpm for various reasons, tire wobble, axle angles, spring flex under evasive driving, J-turn crashes... etc.

If you get it to drive without the vibration at 50 mph, and save $2000.00, it might be the best you can do, for now.

My 2 cents.

This will probably sound a little crazy, but I owned a Suburban one time, that had a 4 inch lift with 35s, and I had a surging vibration at 45 to 55 miles an hour for probably about 6 or 7 years, and could never figure out what was wrong with it. I replaced the entire drive train from the engine back over time rebuilding the truck. I eventually put a new front axle on it, for other reasons, and the vibration went away, come to find out the front axle was bent, and I never even knew it.