Need your help

[The one]

I have a 1971 GMC 4x4 shortbed. When I got the truck 6 months ago it had a problem with the front end. When I apply the brakes from 50 to 60 mph it seems that the front end moves from right to left violently. I can't seem to figure out what it is. When I got to truck it already have a 4 inch Rancho suspension lift, springs in front and blocks in the rear. I have changed the following, front leaf spring bushings, u-joints, steering stabalizer, front and rear shocks and all new grade 8 hardware. I've replaced the rear brakes to include drums, brakes and all hardware. I've checked the wheel bearings and they are fine, I jacked up the truck and don't have any droop or anything loose. What could it be? Any suggestions will help, this is driving me crazy. Thanks