Circling back to update with the solution. (because I hate when people do that)
It turned out nobody checked the lines and when the previous owner lowered it, nobody realized it was linking the lines.
When it was on the lift and the suspension was at max travel, everything was fine, but when it was on the ground, especially when the bed had even a LITTLE weight in it, the soft rear lines kinked something awful.
They'd let fluid in the first time you hit the brakes, but the fluid couldn't get back out, so it held them.
I realized what it was after trying a bunch of other diagnostic methods, but a simple unbending the line with the truck on the ground resolved the issue immediately and entirely.
Thanks for all the advice, folks.
