So here's the story.
decided to do the late model brake upgrade. bought the calipers, BMC, rebuild kits, new rotors, etc. Everything I'd need to do it properly.
conversion took a bit longer than expected and I finished this morning. rear rotors were delivered as wrong part, so i just left the old ones on til replacements arrive (no biggie, same part either way). went for a test drive and they seemed to rub a little. Looked this up and it didn't seem to be a problem. Apparently this happens with new pads, and should clear up fairly quickly.
 
Head out, about 2 k's up the road I notice that it's struggling to make power, and the brakes are holding more. Another 1k I stopped and all 4 wheels pretty much locked up there and then. Pads smoking and car stranded.
 
Anyone EVER experienced something like this? I'm totally stumped as to why they packed up so bad on me.

You used old the old smaller and thinner rotors with the newer calipers on the rear?

They changed the Rotors on the rear? I wasn't aware of that. I was under the impression that the rear calipers, rotors and pads stayed the same from early to late.
So, yes

Yes, they changed both yhe front and rear. So that is most likely the problem. The pads probably arent wearing evenly for starters and the caliper is yo suit a 22mm disc not 16. Which means you may have pushed the piston out further than it is supposed to.

would that also explain the fronts locking with the rears? all 4 were stuck when i stopped the car

Be also aware that later rotors are larger diameter as well as thicker both F/R, compared to early model rotors.
 
Early front rotors are 258 x 25 mm. Later ones are 275 X 30 mm
 
Early rears are 262 X 16 mm. Later rears are 280 X 22 mm

Cheers for the info! Probably should have known that BEFORE attempting the conversion myself. The correct rear rotors will be arriving tuesday, at which point i'll be able to eliminate one issue, and maybe the problem will resolve itself with a fresh flush too!

Did you go for a later model booster as well? Toyota changed those as well and for your larger brakes to work properly you'll need that too.
One from a 92+ car would be ok but the best one to get would be from a 94+ car, but don't forget to also fit a vacuum check valve to the intake manifold.
Later boosters don't have one on the booster body like the early ones.
 

The rotor issue wont effect your brakes being permanently applied. Basically your car is always applying the brakes hence why they started smoking after 2km's then locked up after 3km's. Before even driving off after your rotor change just put the car in neutral and see if it even rolls on a slight hill or if you can push it on flat ground then work out if all 4 calipers are locking or just one. If all four are being applied it will most likely be the pushrod in the master cylinder being stuck too far forward. 

Well I won't be looking at it til Tuesday when the proper rear rotors arrive, but what could I do about the master cylinder if that is the case? Of all of suggestions I've had, that one seems most likely to me.