I have an '06 2500 6-speed w/ an Edge Juice on it. In the past (under stock tires), I drove it pretty hard. I now have 35" Pro-comp X-treme AT's on it and don't drive it nearly as hard as I used to. Aside from the Edge and tires, the truck is stock.

I'm going to try my best to describe the problem that I'm having. Just recently, under initial acceleration (generally 2nd gear starts), I can feel something in the drivetrain shift. Honestly, it feel like it's coming from the rearend, but I can't be certain. It almost feels like something moves. It's only for a very quick instant when accelerating. However, this morning when I was backing down the driveway (steep driveway), I could feel the same shifting feeling when my back tires hit the level road surface. However, this time it felt like the movement was directed back towards me.

Anyone had this problem before or know things that I can check? I'm about to service the rearend fluid in the next few thousand miles. Any help would be much appreciated!

Oh yeah, no sounds can be heard when this happens. Also, it seems like I feel the shifting through the pedals.

 

Check your rear U-joint for play

 

If it was a u-joint, wouldn't I have some sort of noise and shouldn't I feel a u-joint all the time?

 

I would start with a general inspection of the drive train including springs and their mounts, U-joints, transfer case and transmission and their mounts. Mine does some really strange things when shifting while going over choppy bumps in the road, kind of similar to what you describe, but it has done it since day one.