I'm stumped...Yet another brake problem

[Madbomber88]

Ok, I'm stumped and need help with my 72 K20.

When driving on the highway, it will start to studder (can't find any coincidence as to what causes it, it isn't due to pressing the brakes), while it is doing this, if you hit the brakes, the whole truck shudders like crazy, and it feels like there's no vacuum in the brakes. Then after 40 seconds or so it goes away.

So, I just put new rotors, calipers, pads, stainless steel brake lines and hoses, and a new proportioning valve from GMC Pauls. None of that worked.

The rear is a brand new 14bFF, with all new brakes (still drums) back there too. Before I did all the work to the front, if it was acting up and I stopped, the rear hubs were really hot, but is that just normal? Would a problem with the rear brakes cause the truck to shudder so violently?

Any ideas?

I'm going on a camping trip in two weeks and have a 5 hour drive, I need to solve this.