I effectively lost my brakes a couple days ago coming back from Sumter. The drivers front would lock and skid, the passengers would brake up to a point then not engage to keep up stopping force with the opposite sides. I almost spun the car the first time it happened.

I don't know whether driver's side is just too stoppy, or if passengers is being lazy. Can't determine it. I get what seems like uniform braking power for a section of pedal travel, but mashing it makes me tug hard to driver's side, more slides that wheel and locks it up.

Things I know are wrong in the affected area: presently a bad vacuum hose on the brake booster. Working on it. But as far as I know, that should cause uniform weakness/difficulty, not just on one side.

THINGS I HAVE RULED OUT BY DIRECT EXAMINATION:
Stuck slide pins, all four
Exterior damaged hardware (visual of all soft/hard lines calipers rotors brackets)
ABS interference (pulled fuse)
Stuck or damaged pads/shims
Frozen pot (they do on pots on both sides seem willing to move and I can overextend them with the pedal)
Fluid problem/air in caliper (bled good color/rate)
Misaligned brackets (they are imperfect but not to this extent)
Debris under pad

What do I do other than just replace the entire system (possibly including hardlines)?

Suggestions and refutiations:

Could be some trash in the proportioning valve.: Proportioning valve flow is good to and from from all ports

side pads or rotor are substantialy more worn then the other side: Even braking until slamming on it a little. No drag no sticking no uneven wear. Happened suddenly.

Just emptied the passenger side ("lazy") caliper, floating rust in the liquid but no debris. Popped the pots out, toothbrush, checked the rings, all's well. Flows well, smooth, no obstructions. Line runs good. If its the "proportion valve" I cant figure out how it'd be that. It could be debris in the ABS distributor but the out line flows well too. I. Have. No. Fucking. Idea.

Hard braking could loosen up rust in the lines and or restrict flow

Brake fluid has the same chemical that is in coolant and soaks up water like a sponge. Pressure bleed the entire system and start over with fresh fluid. Might want to go ahead and do all calipers the way you did that one. I am sure that it is due to water absorption in the system. What happens is when you brake hard the water reaches boiling point before the fluid does and wherever there is more water will more likely apply more pressure causing the pull concern.

Sure, but it pulled the next day after sitting and cooling all night, as well as at very low speeds with sharp braking. There's a pressure distribution problem, but I can't seem to get the damn ABS lockbolts off, they're made of like plastic or something and just round off with no effort...