This road vibration developed over a few hundred miles and it very pronounced now. What I have is a '91 B250 cargo van with 133k miles, 318/5.2L, A518 trans and 8 1/4in rear axle. The vibration is worst at 50-60mph and it does not matter if it is in OD or 3rd or 2nd gear. I put the rear on jackstands and ran the thing in 2nd or 3rd gear and found no noise with the stethospoce place on any part of the axle or differential, or differential cover. Still I did find the diff oil very low (odd considering no external leakage) and so changed the fluid. Found no damage on the gears other than the odd chip here or there, and minimal metal flakes in the oil (not great, but certainly not bad). No change to the noise so I moved to the driveshaft.
I see no missing weights on the drive shaft but it has a noticable "hop" that seems to coincide with the vibration. I replaced a sagging transmission mount which did reduce the vibration quite a bit, but it definitely did not fix the problem. The U-joints have no grease fittings so I have no way of seeing if they are the problem without pulling the driveshaft, which I'll suppose I can do next. I'd like to get some opinions though from more experienced hands. I'm experienced but 90% of my work is on motorcycles and some turbine engine experience. On cars/trucks, I only fix what breaks and I've never encountered this, so it's new to me.
Also, I did look at the obvious and there is no tire about to separate it's tread or loos lugnuts.If it's not the driveshaft I worry it may be in the trans, and I know very little about auto trans aside from general maintenance. I was just aboutto sell this thing when the problem arised, and I'd hate to sell it as a mechanic's special if I can find something easy.