I have a rear end vibration/shimmy in my 2000, Nissan, Frontier, 4cyl, 2wd, pickup that I have been battling for years to get rid of and it is driving me nuts!. I have replaced the 3rd member (needed to be replaced due spider gears), transmission (needed to be replaced due to wrong lubricant), shocks, new rims&tires, machined the drums... Most recently since yesterday, brand new axle shafts, bearings, shims and seals... Now after just spending $1,100 the vibration is still there (go figure!) and I now have a NEW grinding noise from the rear end that is there all the time, but goes away if I pull the e-brake, or apply the brakes?.

I have had the u-joints, pinion angle, and the axle thrust alignment all looked at and they tell me they look fine.

ANY more suggestions or insight would be GREATLY appreciated!

By the way I took it to the Nissan dealership willing to pay the $89.00 diagnostic fee for them to tell me what it could be and they couldn't... They said they sure felt it, but couldn't pinpoint it... That's where the suggestion of a bad bearing or slightly bent axle shaft came into play. Thus the $1,100 in new shafts, seals, shims bearings, labor and new grinding noise.

Wow, I am sorry to hear you paid all that amount and it still not fixed,
Auto zone does dialnostics for free, see if you have a store near you.
If the dealerhip did not fix the problem, they should take the hit on what they have done, has the problem still was not fixed.
I had a taurus, and many many many problems, the dealership would say this is the problem and when they try to fix what they told me and it still was not done, they took the hit until it was complete...
I hope this can give you a starting point to try, best of luck.