Hey chaps

I have a 2wd cossie 1990. when i bought the car i had a lot of transmission vibrations. I have been working on trying to make it a smooth car to drive.
Have had a new clutch assem, balanced the flywheel. taken the rear axles off re greased the cv joints and had the drive shaft checked for uj's and balance.
Its getting better but still has a annoying vibration at approx 3000rpm. sounds like the whole back end of the car is vibrating at this rpm. happens on acceleration and deceleration... to the point i feel like selling it and starting again. I am sure the diff is quite worn cause there is quite alot of slop when accelerating and then quickly lifting off. so im going to try another diff, problem is there arent any over here! so can anyone tell me if i get a xr4i or xr4x4 diff head and axles (bit more common) will they fit into the back of this car? what is interchangable with these sierras. tried buying a saphire 2.0 twin cam diff which would fit but the flanges are alot smaller than what is on my axles. then thort i could just use the cossie stud axles but ones longer than the other. ahh dam cars. comments would be cool