Just thought I would post my situation today. I came back from driving down a dirt road and sprayed down the truck after I got back. Coming back from the car wash I heard this really loud squeal coming from the rear end. I figured I got a rock under the heatshield. I looked at the rear disc and found the rock wedged between the caliper and rotor! (on the outside) I don't know if having aftermarket rims had anything to do with it, but it made it easier to remove the rock since I could see it without removing the tire. I have never seen a rock get stuck there before. I think that was some kind of fluke.

Used to do it constantly on my 76 GMC on the front. Did it fairly often on the 93. These newer trucks seem to do it a lot less, only done it a few times on this truck.