All right I'll keep this brief as I can. Either the front or rear brakes are locking up, it feels like the rear but honestly I'm not really sure.

All four brakes have been disassembled and reassembled, cleaned, and adjusted. The rear axle seals are NOT leaking. The shoes are fine and installed correctly. Brakes have been flushed and bled. No sign of gear oil. E brake line has been adjusted.
Seen Toyota and my normal mechanic, neither have any idea what is wrong. It has improved significantly after my mechanic cleaned them, but it's still there.

And just specify, when I say lock up I mean right before I come to a stop the brakes LOCK and she stops hard. (Sometimes setting off the antilock) After I have to keep tapping the gas until they release. I can go a whole day and brake as much as I want and it's fine, and then the next day every single stop sets off the antilock brakes. I thought it might be heat, but I've had it to where I couldn't even back out of the garage when she was cold.

Any kind of help or knowledge that anyone has would greatly be appreciated!!!

Heat. Everything creates heat. Flir infrared works best, also expensive. Infrared (IR) Heat gun cost around $20 +/-. Start checking brake rotors, pads and caliper. Usually the sun side of a vehicle will be 10deg warmer.

IR gun very useful for ujoints, radiator, A.C., diff, transmission. I typically carry one in every vehicle.

Do you have an early 3rd gen with no ABS? Could be a messed-up LSPV causing excessive rear bias.

My 1996 had two similar issues. One of the issues was a warped rear brake drum. So at very slow speeds and slight brake pressure (carpool line) one of the rear wheels would lock up. You could verify by pulling the e brake at 45MPH and check for vibration. However you did say that everything has been checked. My other issue sounded exactly like yours. For me, sometimes when the ABS had been triggered one of the front calipers would lock up. And it would not release seemingly until it felt like it. I was able to manage the issue for a year or so. When I upgraded my front calipers to the 231's it repeated the issue immediately and it was not manageable. I cycled the power to the ABS pump and it released. I then removed the ABS fuseable link and have ran without ABS ever since. Not sure what year you have and I know the 01-02's have the big fancy master cylinder/ABS accumulator setup. My guess is it is an ABS pump issue.

Okay, what am I looking for in the rear? And also to mention again, she can do it when she's cold.

It is a 2000 4x4 SR5, sorry I should have mentioned that.

I've wondered about trying her without ABS but...that also sounds dangerous as shit. Is there a way to recalibrate the ABS? I know on 2nd gen taco's you have to calibrate or reset something otherwise it will brake crazy like after doing an alignment or changing the tires.

Only dangerous if you need it! Which I did a few months later in the rain. Exited a wet road into luckily a grassy ditch due to a stopped vehicle on a 2 lane highway. Without questions the lack of ABS caused me to nearly have a major accident. I was lucky and drove away completely unscathed.

Shit, yeah no way I'm doing that...haha. have you ever thought about replacing the ABS system or adding a switch?

Is there any remote possibility the rear shoes could have been contaminated by oil or grease in the past? Because the sudden grabbing of the brakes is exactly what would happen. Try looking closely at the rear shoes and examine the surface. If in double you can let the brake shoes sit on a hot plate or over a fire to burn off any contaminants.

Since I've no idea what their history is or how old they are, would it be worth it to just swap them? How much do they go for?

ABS is for pu$sies. Def not needed on these rigs as the ABS system is archaic anyway.
Sounds like you have a sticky piston, weird that you cant feel what wheel it's happening on.

Do you know if the rear shoes or drums have ever been replaced? There have been lots of reported problems with aftermarket parts for the rear brakes, mainly the drums and shoes.

Have to agree there. I think maybe the newer 3rd Gens with the traction control might be better but the ABS has cause me to slide into intersections when just applying normal brake pressure. I hate it.

For the OP, next time it locks up you need to figure out which brake feels overheated. Really hard to diagnose without it because fronts are disks and rears are drums and they work differently.

During my drive today, I'm fairly certain I felt it in the rear. Plus the selling dealer mentioned the rears were being fixed. So that must be it.

Nope, I'm the 2nd owner haha. I can look at replacing them, but after doing all the other refresh stuff I'm strapped for cash now. Have to go back to one part per month... Ugh
Maybe they put the wrong size shoes on? But I feel like two different mechanics would have noticed that?

I've never heard of ABS causing so many issues, goodness.
I can get a temp gauge, but they can lock even when she's cold, if that matters. I'll see if I can get the gf to remind me to record it.

OEM shoes & drums should take care of it.

The theory behind the temp gauge is the side that locked will be hotter than the other side. Drive around and get it to lock up/stick and then temp each rotor/drum and compare each side