It does feel like ABS kicking in when it happens, but there are no lights on the dash to go with it. I think. The whole dash could be lit up and I might not notice, since at that point I'm basically looking at what I'm going to hit if I can't get it stopped. It's easy to reproduce so I can sure test it out and look at the dash while it's happening.

The belt is fairly new, and the fluid is full since I just flushed it.

I was able to find internet posts complaining about the cold weather behavior from others that said changing to ATF+4 makes it better. So maybe the pump is OK after all.

I'm about at the point of just replacing everything in the system to be sure whatever is failing is fixed.

FWIW the truck has < 80k miles on it.