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Hoping that somebody else has some input or experience with this but basically here are my symptons.

My truck is an 04 z71 and lately I have been getting a sporatic clunk at right around 55mph. It doesn't happen all the time and the intensity of the clunk is different each time I hear it. It seems to be coming from directly underneath the drivers seat. Today I crawled under it and didn't notice anything lose but when I was poking around I spun the front drive shaft just to see if it had any play in it and it sounds like the transfer case is making a very small grinding noise, but could just be gears moving inside, I haven't checked the fluids yet, didn't have any tools with me or fluid to do it. U-Joints on both drive shafts were replaced last year with spicers. I can both hear it and feel it when driving. Anyone have any ideas on what else to check? Hoping to not have to replace the transfer case on this thing...

First thing to do would be remove the rear drive shaft and grease the splines on the shaft inside and out. That's a common problem on the newer trucks.

I have had the driveline clunk noise ever since I got the truck, even after they greased splines when they installed new U-Joints. I also have the G80 locking rear end and have grown accustomed to noises from both of these systems. From the best way I can describe the noise is that it sounds like something is hitting a wheel every so often with a piece of something metal and it clunks but the weird thing is, is that it sounds and feels like it is coming from somewhere near the drivers seat.