Another thing to check for--if the exhaust pipes touch the frame or body anywhere,that'll transmit a heavy vibration at certain RPM's...
I recently replaced the front pipes on my diesel,and man,with one touching the frame just slightly on the right side,it sounded like I had installed solid steel engine mounts!..I had to heat the pipe up and pry it away to gain 1/2" of clearance..
A van a friend had sounded like it needed a rear end,it had what seemed to be noisy pinion bearings--the ring & pinion looked good,no excess backlash,so he replaced the pinion and diff carrier bearings--and it sounded exactly the same..it was like a high pitched whirring noise with a grinding vibration..
Only other thing he had done to the van was replace the rear leaf springs,and he did remember there was some kind of plastic or teflon "shim" between the spring pads on the axle tubes and the springs..
He decided to go through the agony of taking the u-bolts back off,and installed some new ones from a spring shop--and ta-da,the noise vanished !...