First Drive after building, Major shake

UPDATE:
So we sent the original brake booster out for rebuild (Karps in Upland Ca. great people) and returned the one we bought from Parts Geek. We also replaced the Master cylinder since it was the only thing we really haven't replaced.
We got the brake booster back yesterday and installed it and the master cylinder last night. I pulled it out of the driveway and did a couple of laps around the block just see how things were and everything seemed fine; the pedal was a little low but we will bleed the brakes again this weekend and I think it should fix that. So I chanced it and took it for a longer drive keeping it at low speeds. Again, everything seemed fine the brakes were working well and the car was driving well. I then decided to get her up to speed and see about the shake........and nothing; all was well. We drove her for about 20 min getting it up to 40-55 mph and NO SHAKE! So at this point I drove her to the freeway and jumped on. I cruised in the slow lane for about 4-5 miles at about 50-60 with no shake and then took her up to 75-80 for another 5-10 miles and it drove very nice. Alignment was nice, the braking was nice and NO SHAKE. We jumped off the freeway and took side streets home and everything was nice and as it should be. She pulls a little to the left when braking and I still want to bleed the brakes again but I think the shake situation was in the braking system. Could a faulty or bad booster apply the brakes enough to grab on its own? My thought is while driving at higher speeds could the engine be increasing vacuum in the booster which would apply the brakes???? Because in our shaking situations with the Parts Geek booster, every time we applied the brake it would go away thus bleeding off the vacuum??? Just a thought. I'm really just happy that the shake is gone and the brakes are almost (the pedal is a little low) where I think they should be.

Thanks to everyone for the support and help! There is no better site in my opinion than this one. The best information, knowledge and most of all understanding of what owners go through. I should add patience for a Chevy guy trying to build his first Mopar!

Thank you,
Matt