Load Sensing Proportioning Valve. Look for the little rod coming off of your rear axle housing on the driver's side, and going to the pasenger side frame rail. It attaches at the LSPV.

The valve is supposed to adjust the amount of rear braking bias according to the load applied, but it has limited travel. The other limitation is that the springs sag less per pound once the pack rests on the overload leaves. If you hit the overload leaves, you now have less rear bias in the brakes than you should.

Under normal circumstances, this is fine... But when pulling a large load, you need all the rear braking it can handle. Make sure to re-attach the rod once you are done. If not, you will hae a tendency to lock up the rear brakes, potentially swinging the rear of your truck around in an emergency stop.