Shutter Noise from Drivetrain

[Enrogersgm]

Just today, I started to have issues on my 1997 K1500 (4.3L 5sp Manual). I drove it about 20 miles in some pretty decent stop and go traffic when I started feeling this shutter out of the drivetrain. It seems like a moving gear was hitting a gear that was stopped, or something similar. I initially thought maybe a U-Joint was bad but I checked both sides of the driveshaft and it was good. My clutch was replaced 15k ago and my truck is sitting on about 133k. The rear differential fluid has been replaced and the transfer case and transmission do not leak at all. Any idea what would cause this? It seems to do it at low speeds, like when I'm slowing down after going 45mph or so. I do know there's a little slop in the drivetrain, but it has been that way since I've owned the vehicle (117k). I can't tell if it is a problem with the rear end, transmission or transfer case?

I've had a bad u-joint on a truck before and that similar "slapping" noise you would hear from that sounds sort of like the issue I'm having now. Any advice for places to check would be appreciated.