Recently, I had a reliable local shop change the front and rear diff fluids for me. Not too long after that I had a few situations where I would be in reverse and get a pretty solid "clunk". Each time it happened, I immediately put it in drive and moved forward to make sure I didn't hit something; after that, I would have no issues driving it.

Eventually, it got so bad that I could not get the truck out of my garage. The owner of the shop came over with one of his techs to investigate. After about twenty minutes of moving the truck forward and aft, they determined that they were pretty sure it was something in the parking brake. That's what it turned out to be. The left parking brake shoes had corroded to the point that the pad material had come off the shoes and would jam things up in reverse.

So... just a heads up for you, if you come across something like this.

 

Awesome and thank you for sharing! Frustrating and not easy to find these things. Nice to know it wasn't something major or costly...