howdy, i heard this is a common problem.
i have a 1991 dodge ram d150 3.9l v6. has about 150000 miles runs fair enough.
ive been chasing this vibration during highway driving for quite awhile now.

i replaced my control arms lower and upper with all new bushings and ball joints, new coil springs, new tie rods, fresh rotors and new front wheel bearings, new six leaf rear springs with new bushings, everything aligned. then driveshaft got balanced and new weights installed yesterday, new u joints installed. drives much better than it has in years but yet..

still getting this shake/vibration during highway travel at speeds between 67.5-72.5 mph (speed is off abit so it may be 5 mph higher than the actual reading)
anything lower than 65 seems fine and smooth and above 80 is great as well.

only things i haven't worked on is the engine, transmission and rear axle. never done the bearings in the axle shafts or front or rear bearings in the diff.
if you worked on rear axles or transmissions on this model truck id like to hear what you got to say

 

what about rear drums? do they still have their weights if they had them? i don't know if you can have those balanced or not but could maybe cause a shake if far enough out of balance.

 

didnt even know rear drums had weights, ill see what the manual says.

 

not all drums are weighted. it is just like rims and tires. some may need weight while others may not but most usually have a pretty big weight welded to them