pulsing vibration

[crewcab higgy]

My truck has had a slight pulsating vibration at highway speeds hardly noticeable. I put my new motor in and drove it last night and the vibration got worse. Before it had 2 worn out motor mounts and now it doesn't but idk what it could be. It happens on the throttle at highway speeds in third and feels like I'm hitting the rumble strips in 1/2 second on off pulses in 3rd. In second it vibrates bad at above 2500 rpm.
It's bad enough I'm considering towing it to a shop but I'd like to make sure I'm not overlooking something.
Also 454 th400 dana 70 combo

 

[Wildjoe330]

Bet you have a 203, I am going to the 205 soon my 203
(past full time worn out Chain) vibration goes away.
I'm adding over drive in a 700r (core) trans. that came
with my 205 and adapter. I will probably trade in my 700r
for a rebuilt one now that will bring it up $500.

I don't think the 203 chain is getting the lubrication that it
should be getting. every 200 miles run it 10 in full time

 

[CSFJ]

Bet you have a 203, I am going to the 205 soon my 203
(past full time worn out Chain) vibration goes away.
I'm adding over drive in a 700r (core) trans. that came
with my 205 and adapter. I will probably trade in my 700r
for a rebuilt one now that will bring it up $500.

I don't think the 203 chain is getting the lubrication that it
should be getting. every 200 miles run it 10 in full time

His info below his screen name says R-3500. 2 wheel drive. 1 ton. No transfer case involved.

 

[CSFJ]

My truck has had a slight pulsating vibration at highway speeds hardly noticeable. I put my new motor in and drove it last night and the vibration got worse. Before it had 2 worn out motor mounts and now it doesn't but idk what it could be. It happens on the throttle at highway speeds in third and feels like I'm hitting the rumble strips in 1/2 second on off pulses in 3rd. In second it vibrates bad at above 2500 rpm.
It's bad enough I'm considering towing it to a shop but I'd like to make sure I'm not overlooking something.
Also 454 th400 dana 70 combo

If the motor mounts went bad, have you checked the trans mount also? Mounts tend to weaken the remaining good ones when they go bad, because it adds more load to the remaining ones. Also what kind of shape are the U-joints in?

 

[Wildjoe330]

77 K20, 50 to 65 mph pulsating Vibration, Was Full Time, Replaced front axle u-joints, Cab Body Bushings, Front Wheel Barrings gone thru, Upper Lower ball joints, New front pads, New Lockouts, Used Part time Kit, Rear drive line Balanced New U-joints, New Tires/Rims Balanced, New Transmission Mount, 350, TH350, NP203, 7/14 10:22

 

[Keith Seymore]

77 K20, 50 to 65 mph pulsating Vibration, Was Full Time, Replaced front axle u-joints, Cab Body Bushings, Front Wheel Barrings gone thru, Upper Lower ball joints, New front pads, New Lockouts, Used Part time Kit, Rear drive line Balanced New U-joints, New Tires/Rims Balanced, New Transmission Mount, 350, TH350, NP203, 7/14 10:22

"Pulsing" vibration is caused by TWO root cause issues. As the (sinusoidal) amplitudes add and subtract the disturbance builds and decreases causing the pulsing sensation. Killing either one of the root causes will make the pulsing go away.

4th order of engine (firing frequency) and 2nd order driveline (u joints) are often the culprits. I'd spend some time rechecking those motor mounts (overtorqued? grounded out?) and U joints (working angles at speed, etc) to see if you could make it change. If you can make it better, or even worse, it will tell you if you are on the right track.

K