Is it usually on the first few brake applications? These trucks lock up the rears hard the first few brake applies if it is slightly humid or you've driven through water, etc etc...
Your drums flash rust while they set and the rust makes for very grabby brakes the first few applies, usually by the third apply it is knocked off of my truck and the brakes work fine.
Like the others said though, if your wheel cylinders are leaking brake fluid or if you have a leaking axle seal, they will lock up also. To be safe I'd pull the wheels and drums, have fun they are probably stuck pretty good. If the pads aren't oil soaked, knock the glaze off your drums with some crocus cloth, clean up the drum and brakes with some brake kleen and adjust em up. Unless you haul alot, your rear brakes will last a very long time, the braking is devided 70\30, the front getting most of the work, and if you are empty the rears do very little unless you have a leveling or lift kit to put more weight to the back...., this is also the very reason the rears lockup so easy. The fronts don't have any anti lock system at all and you probably will never lock em up unless you are on snow and ice or dirt roads...
