my 2000 gmc 2500 4x4 jumps when braking sometimes hard, sometimes barely?????

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OK, I have a 2000 GMC SIERRA 2500 6.0L V8. I just recently purchased this truck and have noticed the brakes being a little jumpy occasionally. Sometimes very bad, sometimes very little, and sometimes perfectly smooth. I pulled the wheels off and checked all of the rotors ( front and back ), and they seem to be worn down (1/16" MAX) (more like a 32nd). Anyways, they look like they have very little wear. I tried to use calipers to check for warping while spinning them and it seems that they are fine. (im no expert though). My problem with the whole situation is if my rotors were warped or worn I would expect my truck to jump everytime i brake, instead, it only jumps (or i notice it jumping the most) at lower speeds 35mph or less, and 3-4 times out of 10 it brakes perfectly smooth which leads me to think, something else could be going on. Has anyone ever heard of this or have any knowledge on this scenario? anything at all would be helpful. thanks