SHAKE... While and AFTER I hit the brakes

[epfatboy]

Got an '86ish K10 with like a 3" lift an 33's.

Had the truck for 3 years done nothing to it but change plugs and oil so something was bound to start complaining.

Took her for some errands and noticed a small shake, thought it might be the brakes but nothing to worry about yet

The next ride was eye opening, got about 10 miles out and hit the brakes at around 50mph and she started shaking hard, let off the brakes and the shaking went away. I've driven my share of **** boxes, this wasn't a shake you ignore.
So now I'm heading home and need to hit the brakes again, starts to shake hard let off the brake and she kept shaking for about another 30 seconds... WTF! Sonow the speed does not exceed 20mph, still shakes but doable.

Got it up in the air,

Nothing sticks out yet but haven;t taken a wheel off yet 'cause it's 900 degrees out today.

Any ideas of what to test and how. I was thinking about rebuilding the suspension components minimally anyway but really not sure where this is coming from and I'd like to figure that part out before I dig in.

Is there anyway this could be coming from the rear brakes? Sorta felt like it was the ass end hoping all over the place but I can't rule out the front.