So here's the history:

When I first got the truck a year and a half ago, I noticed the RWAL
ABS light would come on occasionally, but only when applying the brake
in reverse.  There would be pressure at the pedal, but not at the
brakes themselves.  Before long, I noticed that pumping the pedal a
couple times, even while in Drive, would cause the same thing.
Pressure at the pedal, but the vehicle lerches forward.  I know you're
not supposed to pump the pedal with ABS, but when inching along in a
traffic jam, it is inevitable.  I tried disconnecting the ABS fuse,
but that didn't change the problem.

I also checked the combinantion valve to make sure the valve stem
protrudes when the brakes are applied.

Also, somewhere along the line, the truck started showing the clasic
signs of warped drums: vibration/pulsation while braking (even at very
slow speeds), though not side/side vibration.  I went to get the drums
turned, and the parts shop salesman said they were barely out of
spec.  He turned them, but that didn't help.  I gather that he
completely messed up, but perhaps I reassembled the brakes
improperly.  Before I'd even dropped the truck back on the ground, I
could tell it was still bad, because the drum would only rub up
against the shoe for half of the wheel rotation.

I'm off to get the ABS code from the truck.  Will post it when I find
out.  Any help is greatly appreciated.

Nate