Rear lock-up
 

Anybody ever figure out why the rear locks up sometimes when it's cold/wet for the first few stops? I'm sure it will come back around this winter.

Yep, that was figured out a long time ago. It's mositure that collects in the drum on the shoes. When you brake that first time or two it creates fiction and enough heat to dry it out. There's not much you can do for it other than hit your brakes once or twice before you really get going.

Why on F150's???
 

There are quite a few rear drum vehicles out there. The design of the rear drum seems pretty common throughout, why would it only do this on F150's then?
I guess it could be the difference in the ABS.

I don't know why the F150. I know if you search the board you will find more than one thread about this very issue. It's gotta be something with the design of the drum and backingplate.
I know dads 94 doesn't do it and no other vehicle I've drove has did it either.
The ABS sensor for the rear on the 97 sits on top of the rear diff as the rear is only a 1 channel ABS setup. I can't see how it would affect this problem. I guess if it thinks thinks the rear wheels are locking up to fast it activates the ABS portion of the braking system and that would explain the lock/unlock feeling you get. This is just a wild guess.