Sounds like the source of the vibration has not yet been identified - correct?
All stationary driveline components (axle tubes, transmission housing, transfer case, etc.) have been inspected and found to be securely mounted to the frame - correct?
Rotating elements of the driveline (driveshafts, axle shafts, gear sets, etc.) are in good shape, properly balanced and correctly indexed. U-joints are tight and any splined joints can move freely in the axial direction. IMPORTANT - Any bearings that support the rotating elements of the driveline have no excessive play. Correct?
Rims and tires are balanced and also have no "run-out" or "out of round" issues. Right?
I only ask these questions because - a vibration that occurs only during braking is generally blamed on ... the brakes. But sometimes the root cause is actually due to a seemingly unrelated component.
But, at this point - having checked/replaced any other possibilities - it sounds like you are confident that this vibration is definitely brake related.
If truing the drums up does not have any effect, and yet you still are fairly sure it is the brakes; try this to isolate whether it is the front or rear brakes. It won't tell you exactly what is wrong but will allow you to concentrate your efforts on one braking system.
This is not for the faint of heart - especially given the speed at which you are experiencing the shaking:
Before you do this test be sure your e-brakes are in perfect working order. Also, check that when they are operated, braking effect is applied equally and simultaneously to both drums.
Find a stretch of road that is not busy, and bring your truck up to the speed at which the vibration normally occurs.
Activate the e-brakes. You don't need to crank down on them to get a good result from this test. Just apply them enough to feel them gently engage the drums.
This exercise will isolate the rear brakes from the fronts. The same components in the rears will do the braking just as would be true if the brake pedal was used.
But, since they are not activated by the e-brake system, the front brakes will not be part of the test.
I'm sure you can see where this is going. If, when using only the rear (e-brakes), the abnormal vibration is still present...the problem lies in the rear brakes.
And, conversely, if there is no vibration using just the e-brakes...you can cross off the entire rear brake system as the cause, and concentrate only on the front brakes as the source of the problem.
