rear end vibration, pump not shutting off

I have a 99 silverado. the rear end starts bouncing when the brakes get warmed up. I have replaced the pads and tires but still no change. Also, last night when I shut my truck off I heard a humming noise from under the truck. It is a little box with hoses coming out of it. I had to unplug the wires to get it to shut off. Can anyonehelp me? Thanks

sounds like your rear rotors need turning/replacing. i would suggest replacement.

i dont know what the black box with wires would be. where exactly is it located??

also,does your truck have duel fuel tanks??

and is your truck a new or old body style (1999)?

no, I have a single tank
It's not neccessarily a box. It looks like a pump of some sort. it is located on the frame under the truck, almost directly under the drivers seat

That box making the noise is your anti-lock brake controller/valve. Sounds like it needs replaced. If im not mistaken its pretty expensive. Im also guessing yours is the new body. Hope this helps.