Sound Coming From Rear End

[Mojavered]

First, I have an 06' Sierra 1500 4X4 SLE2. It has 33" tires on 20"X10.5" wheels. Not too big, but heavy and make a slug of a difference. I had the gears changed from 3:43's to 4:10's with a swap on the G80 for an auburn unit. The rear gears are OEM. It has dual magna flow exhaust, CAI, and a performance tuner (not sure it does anything though)

The problem lies here, gears changed. Drove it about 1000 miles, light driving and a trip to Vegas (15.2 mpg) and took it off road once in the hills on a well known hill to make sure all worked. I was just thinking about changing the fluid when I started to get a CLUNK in the rear end somwhere. It happens when it shifts from 1-2 occasionally and from 2-3 95% of the time. I got underneath it and grabbed the driveline and am able to move, rotate only, the driveline about +/-.125". I was not able to move it up and down, side-to-side. I took it back down to the place that did the swap. The took the drive line out and seems that the U-joints are fine. They then grabbed the propeller shaft and started to rotate it back and forth and it was making the similar noise I am hearing. They said that the play was normal and I went over to another truck they had there and it too did it. They said that I just never noticed it before but this is BS. I guess I am kind of peculiar and notice things that even a deaf person would hear. Any ideas on what I could look for? I am getting ready to take it to another shop, but would rather not. Sorry for the long post and thanks for reading!

[Mojavered]

When checking backlash, the only way to do this is to open the housing up and see gap or lack of gap between the ring gear and pinion. Is there any way to check without getting into it? Thanks!