2001 F150 4x4 8.8 rear end 350 ring

So I just rebuilt the rear end with new gear, pinion, all 4 bearings, spider gear and clutch pack on limited slip., axle bearings, u joints

I picked up a problem I had in the past which is rear truck bounce when I brake. Its a hard bounce when I brake normal. I have to purposely brake hard to try and stop it.

This happened before a couple of years ago, it was caused by a differential seal leak that I didn't know was leaking until the bounce started. I put on new brakes, rotors but it ended up being because it was 2 qts low on fluid. After filling it, the bounce went away. I just rebuilt it because of bearing noise.

I can't for the life of me figure out how or why the rear of my truck bounces, not vibrate but bounce when I apply the brakes. Oh, tires might have 3,ooo miles. It's very frustrating. I marked my drive shaft for re-install and even put on new front and rear u-joints. I torqued everything to specs and verified my gear pattern multiple times. Other than braking, everything else is fine. No noise and it's smooth.

What the heck could cause a serious bounce?

 

Bent axle?

 

Wouldn't that vibrate all the time? I didn't hit anything.

 

I have seen just a slight tweak be aggravated when the caliper locks on the pads. Is it just while braking? What I have done to see maybe what side is doing something, is clamp off the soft area of the brake line to one wheel, drive and see, then take that off and clamp the other one. I just used a vice-grip with two small pieces of wood so not to damage the line. I have even put the rear end on jackstands and had my wife run it slowly up to 40 mph, then gently brake it and then I watch. LOL...she liked that better than "pump it twice then hold it to the floor"