Found something out today that I hope helps someone else
I have got a Series 3 Stage 1 V8 with an LT95 and a Detroit in the rear diff and over the last 12 months or so the gearbox back lash has been getting annoying despite the box being rebuilt at the start of that time.
Well I recoed myself another Centre Diff but havent had the chance to put it in but I noticed that the rear tailshaft seemed a bit worn. So I bought another rear tailshaft today put it in and its like driving a new car absolutely unbelievable how much difference its made.

So if you excessive drive train slap dont ignore your tailshafts (like I did)