Thumping while accelerating from dead stop

[Elrond the XJ]

Good evening all,

2000 XJ sport - complete stock

While acceleration from stop I have a thumping noise that sounds like it is coming from rear. Does not continue after one or two seconds. It is rhythmic. I have replaced u joints in front drive shaft thus far ( waiting on u joints to arrive for rear)

I have also noticed a good amount of rotational play at t case and rear drive shaft and rear dif. I jacked up the wheels and it seems the drive shaft does not rotate with tires immediately (some play on rear passenger not as much on rear driver) not sure of that is common or not. It has been almost 18 years since I've worked on an XJ (owned a 94 back in 2004).

Changed diff fluid about 2k miles ago 75w90 amsoil and the gears looked good. Noise started maybe a week ago (well after fluid change, maybe I needed to go heavier weight).

Thoughts if I'm going in the right direction?

[Elrond the XJ]

Add - there doesn't seem like a lot if any play in rear DS u joints, a tiny bit at TCase.

[Calvin Bnvds]

Have you managed to solve the issue?