... only under throttle at all speeds. certainly bothersome enough to get rid of it for $500 especially on long road trips. ...
Drive shaft geometry/balance vibrations almost always happen only at highway speeds. If it happens at lower speed, would be more likely to suspect a failed joint.
My first test: Block a wheel, shift the transfer to neutral (this will deactivate the park pawl, so be sure the rig cant roll). Under the rig, confirm that the drive shaft isn't loaded, can rotate it a few degrees. Grab a flange and the shaft on the other side of the joint, attempt to rotate, move, there should be zero play in the joint. Check all four, if any have play, you may have found the problem.
If all check good, get the grease gun, preferably with thick tacky grease, my favorite for u-joints is Green grease. Pump the two joints on one shaft full of grease, if the stuff coming out is black/nasty, flush until you see fresh. Drive the rig, any change in sound/feel? Repeat on the other shaft.
This is the best method I have found to locate the problem. A fresh load of grease will often temporally mute/change the sound/vibration. By doing the shafts separately, will give a good indicator of the problem shaft.
