How much play is normal in the front driveshaft on a '98 Jeep Wrangler?

Have a '98 TJ with 4" suspension lift and a 2" transfer case drop. The front driveshaft pops a lot when the 4 wheel is engaged and I've noticed lately that even in 2 wheel I hear a lot of clicking coming from the front driveshaft. I umed it was the u-joints wearing out, but when I crawled under it tonight I discovered that the u-joints are solid but the driveshaft will rotate at least 1/4" maybe 3/8" (outside surface of driveshaft moving) back and forth. It's in the shafts coming from the transfer case and going into the differential where the movement occurs. Is that normal? The Jeep is on it's 3rd set of 4.56 ring and pinion gears in the rear in 4 months due to improper installation by the shop. First 2 times the gears were screaming on the highway and they told me not to worry about it.. well they self destructed and the shop had to replace them for free both times. Guess 3rd time's the charm because I got it back thursday and the gears run dead silent at all speeds. This has me worried though that there may be a problem with the front 4.56 gears which were installed at the same time the first set was put in the rear 4 months ago, though I have not heard them making any sound it just worries me that there is so much play and popping occurring in the front driveshaft. Any suggestions? One of the next upgrades I'm saving for is a slip yoke eliminator and cv-driveshaft for the rear so I can get rid of the transfer case drop, will this help the front driveshaft's popping at all?