You want absolutely ZERO play in the joints.. The drive shaft will have a little play, but it can't be coming from the u-joints.. You have a little backlash in the pinion to ring, but not much at all.. and again, some back lash out of the transfer case, again, not much.. but you want he joints TIGHT.
If you have even the slightest play in your u-joints they will go south in a hurry..
You have to get under there and really look closely when you inspect them and move the shaft. Sometimes one of them is so tight, that you can't get any play from the other. I find it helpful to jack the rear-end off the ground and have someone turn a tire back and forth, with the vehicle in gear, while I get under and watch the joints for play.. Just reaching under and trying to turn the shaft will rarely show you the whole story.