Make sure the brake shoes return to rest on that top pin when the brake peddle is released…A wheel cylinder can seize up, allowing a rear-facing shoe to remain in contact with the drum. When the drum rotation direction reverses, that shoe can self-energize, be pulled into the rotating drum…Nice pix…That is indeed a standard differential.
Your step by step plan to isolate the problem is a good one…
…and the suspense mounts.
But no answer yet. 
Hey Guys, I’ve been a little under the weather and didn’t feel like laying down on the cold garage floor for the last couple of days, so no movement. I had to order a new diff gasket anyway and figured I didn’t want to put it together without one just for that test in case it turns out to be a brake problem and then I have to bench press that huge chunk of metal in and out of the housing yet again. Especially since after I’ve slept on it, Caddyman’s seizing brake cylinder thought is gaining more traction in my imagination. Looking back it would have been good to have actually watched the brake action with the drum off and maybe a sticky piston would have surfaced.
OK, I’ll be back in a few days probably.
~Dan
Take your time…And use a floor jack to lift that pumpkin back in…
Hi Guys, I bet you thought I fell off the planet. Sorry for the delay but with being ill and other family urgencies, I finally got around to dealing with the truck. And then it took two weeks to get up the courage to write this. So here it is. I’m an idiot.
After finding a used diff for $150 like the tranny guy told me to do, I put it all together and what do you know, same problem! AHHHHH! Rolled the truck up and back, locking up, and took notice of the front wheel. It looked like it was the front locking up now. So I jacked it up and sure enough, it was the front! One of the caliper bolts had fallen out, the bottom one, so when the wheel turned in reverse, the caliper could rotate and jammed the wheel.
My own fixation on the rear was misguided as I associated the “reverse” problem with something in the drive train. But I guess what really threw me off completely was the tranny guy when he had it up on the lift. He said he could feel it catching in the diff. Well, the bolt was $1.50 from Toyota and I got two cuz I figured that other one had gone through a lot of stress with the wheel lock-ups.
Well, I have a spare diff now; in 15 years when I need one, I won’t be able to find one so I’ll just keep it in the shed.
Thanks guys for all your help earlier. I guess I learned to check all the wheels regardless of where I think the problem is coming from.
Take care,
~Dan
Thanks for coming back and closing out your thread…
Nahh, you’re not an idiot. But your transmission guy,…