Rear Differential Slack

I've searched, but haven't quite found the answer.

The details:
1) 1999 XJ
2) 4x4 Auto, 4.0, AW4
3) 109k on the clock
4) Daily driver, stock height, etc
5) D30 front, C8.25 rear

While getting new tires yesterday (Toyo Open Country H/T - 235/75/15), the car was up on the rack, I asked the shop guy to check and see if he noticed anything out of the ordinary that might be causing the rear-end clunking from a stop while in drive - you know, the typical: axle wrap, worn u-bolts, out of adjustment rear brakes, etc. He noted everything looked fine.

As I was standing there, I grabbed the tire with the wheels off the ground to spin it and it had 2-4" of play in the wheel before the differential took up the slack. My question...is this normal? How much play should there be in the differential? Could this be the cause of my clunking from a stop while in drive? I stopped at Napa during lunch today and picked up some gear oil and a new fel-pro gasket to change out the diff fluid after work as it's long overdue.

Any thoughts from the naxja braintrust?
mikeeace:

Depends on how badly the spider gears and pinion bearings are worn and how much backlash the ring/pinion have. The transfer case, park lock, and driveshaft slip yoke + U-joints also contribute a little.

I had the same "play" and the same clunk. It was the rear u-joint, toasted. I was able to twist it a little and push it over about 1/2 to 3/4 inch. Replaced, clunk, play all gone. Replaced the front of the DS joint too.

Changed the diff fluid last night and while off the ground and with the tranny in neutral, I was able to spin the front and rear driveshaft an equal amt - 1/4-3/8" or so... The inside of the diff looked good - although the oil was long overdue to be changed. We'll see how it feels on the road today.