Pulsation While Braking

[SPQR]

My truck is a 1999.5 Sierra 2500 with 4 wheel disc brakes. I would like to figure out why there is an ever more noticible pulsation when braking. It is getting to the point where braking hard at high speeds (like slowing down on the highway unexpectedly) shakes the truck like I'm on a washboard road. I know I have to do the rear brakes. I'm hoping that by replacing the rotors (which I'd probably have to do either way) will eliminate this pulsation. I cannot think of why the pulsation would be so bad considering that the front brakes are in great condition, and the back ones need to be done, and there is no vibration when I'm not braking. Basically, I hope I have it figured out by concluding that one or both of the rear rotors is warped. I pretty much just pulled that out of my ass. Can anyone tell me if those symptoms sound familiar, and what the cause/solution was?

 

Perhaps notable is that when I put on some tires, the guy told me that one of the rims was slightly damaged. I would think that if that were the cause, that it'd be apparent while cruising, and not just when braking, so I kind of tossed that possibility aside.

[8.1HD/ALLISON]

Sounds about right to me. I would replace the rear rotor since they need it anyhow. If it still vibrated I would replace the front.

[Z45]

Rotors are warped. Make sure the wheel mounting surface and the matching rear of the wheel a clean and free of any rust, scale or anything. If the surface is not perfect the wheel will not run true to the rotor and cause this. Also, its a good practice to replace rotors in pairs on the same axle, any difference from one side to the another can affect performance.