I hope its just a u-joint...
 

I have a 03 sierra 4x4 5.3l, with about a 106K on it. I have a feeling a
u-joint is shot, but there is no play at all when I get underneath and screw around with it. I would say it has all the common symptoms - rough shifts from 1st to 2nd and 3rd, slight squeaking noise at low speed, a rumble at high speed, clunk when shifting from drive to reverse. BUT, this only happens off and on??? for example, in the morning it has all of those symptoms, significantly, and by the after noon it may run like its new off the lot (for a couple hours). The fact its only happening a little over half the time makes me more nervous than it does happy. Also, it is a single joint drive shaft, if that changes anything? Thanks.

Ya know, at 106k, it probably wouldn't hurt to change them out anyways...

It's not a very difficult or expensive repair and it'll either fix the problem or rule out a possibility. Have you done the 100,000 mile maintenance on it yet? Gear oil, tranny oil and all that stuff wouldn't hurt to have changed out.

Hopefully this helped and welcome to the forum!
Kevin

I bought the truck about 4k ago, did a all fluids/oils when got it. In the past two weeks, I got two new wheel bearings put in, 1100 bucks, and the next day someone had fun with the breaking of my windows while i was sleeping, 1000 bucks. so im praying to god its just a 50 dollar Ujoint. Cant handle anymore of this sh*t. But thanks for the info!

I had no play on mine when I thought mine were gone. Turns out the one in the rear came completely apart when I unbolted it. So chances are yes, but you gotta take it apart to be sure. If you do replace yourself, you need a torch to heat up the factory nylon otherwise you will be beatin it with a hammer non stop. The new ones use c clips so its easier to assemble.