The most common cause is a tight universal joint, but also look at drive shaft angle. From the rear axle, the drive shaft must go up slightly, and must angle down a little on the transmission output shaft. Those angles are carefully designed in. The biggest offender of incorrect, (reduced) angles is weak springs that raise the axle up into the truck. The second worst offender is lift kits that greatly increase those angles.
There was a service bulletin in the 1990s about drive-line vibration that involved replacing the two-piece drive shaft with a one-piece unit. Years later when the vibration came back, you were to replace the one-piece drive shaft with a two-piece with center bearing. Ride height was still within specs but it solved the problem both times. You might check with the dealer to see if there is such a bulletin that pertains to your truck, but I haven't heard of anything. Have an alignment specialist measure ride height too, particularly in the back.
If the problem does not occur when the engine is still cold, you might consider a torque converter shudder from using the wrong transmission fluid. The clutch plates will chatter as they engage at around 40 mph. The way to verify that is by cruising at a steady speed above the lockup speed, (you should feel it engage and engine speed will drop about 200 rpm), with the engine warmed up, then tap the brake pedal while holding the gas pedal steady. Engine speed will increase for a couple of seconds, then drop back down when the lockup clutch re-engages. If you feel the vibration each time it locks up, suspect the wrong fluid is in the transmission.
Wednesday, January 26th, 2011 AT 5:12 PM