4wd Weird Shake when Braking

[72k20Farmtruck]

Hi everyone, I have an 82 K20 pickup that drives amazing, it has a 99 vortec 454 with 4l80e and Np208 that I swapped in a few years back. Well about a year ago, the 9.5” 14 bolt rear end chunked the gov-bomb. So i had a wrecked van on the property, a 2004 express 3500, I ripped the disc brake rear end out of and swapped into the 82. I cut all the old perches off and welded on new from Barnes 4wd, set my pinion angle, all that.

When stopping lightly, the truck has a very weird up and down shake to it.

I have changed the shocks, turned the rotors (they were new originally) new calipers, akebono brake pads, new wheel bearings, and It still does it. Hard braking is no problem. Stops smooth and very well!

I have a disc/disc prop. valve, a master cylinder for a 2000 c3500 pickup, and the factory style calipers.

Need some advice, it is driving me nuts.