When I first start driving my truck, the first several times (1 to as many as 5 or 6 times) I hit my brakes, one of the rear wheels "grabs" and, if on dirt/gravel, will lock up briefly (if on concrete or asphalt, it will lock and squeal the tire). The problem has been getting progressively worse. I recently changed the rear brake shoes, which were glazed and cracked, but not significantly worn, and the problem went away for a few days, but is now worse than before.

 

There was alot of brake dust inside the drums, but that's to be expected. No oil leaks or evidence of previous leaks. No brake fluid leaks anywhere that I can find -- I park on a concrete slab and there are never any fluids under the truck (except for the A/C condensation - normal in Florida), no rust, no unusual wear inside the drums. There wasn't even a ridge at the edge of the shoe wear area on the drum - something I've seen on some other vehicles, but not on this one.

Also, this is the first time I've had the brake drums off the truck and I've put about 130K miles on it since I bought it used -- it now has about 155K miles on it.