It is often found that the front diff mounts are not holding or supporting the diff correctly. This situation overloads the rear mounts which eventually fail. The problem seems to have little effect on rear wheel alignment, but does have a direct effect on the alignment of the tail shaft.
Tail shaft balancing has always been an issue with Spittys, so if you have an ongoing problem with balancing it may be worth carefully examining the diff mounts.
The test is to jack the car up by the diff and if you can turn the lower mounts easily it needs to be fixed. Unfortunately the new mounts that are now supplied are often aftermarket and too small (not the correct dimensions). Rimmers advertise a special poly set (133568P) which I've found to be good.
My fix with aftermarket mounts was to fit original lower mounts (4ml thicker) and fit 1500 LH distance washers (134603) to both sides.