98 AWD vibrations

I recently purchased a 98 AWD with 90K original miles. I have noticed that at 100KHP when you back off the accelerator, there is a vibration in the drive line. Took it to a shop and after a few hundred dollars, they said the rear shaft was OK but the front was stiff. Wanted $2800. to replace. I went to the wreckers and purchased one and installed myself, but the vibration is still there. Another shop said I need to balance the shafts. Where is the best to invest?