I just picked up a new to me, '91 4 Runner, 3.0, 5sp, 4wd that had been sitting for a few years, so, I drug it home, cleaned it up, and got it ready for a road test, aired up the tires, checked oil, water, etc. Fast forward... I'm on the freeway and up to speed and all of the sudden it sounds like I lost a tire, a little banging and clunking, a high-pitched buzz, and probably other noises too. I was able to get it off the road and as soon as it got to about 10mph the rear tires locked up, I put it in reverse and rolled back about 5ft. and then drove in first without it locking up until I let off the gas and then it locked up again... lather, rinse, repeat for a half mile until I got it to a safe extraction point. There was no noise, no vibrations, on indication this was about to happen leading up to it. Luckily I had just added it to my insurance the night before so roadside was there in 45 minutes. It'll still drive in reverse (what little I did) but wants to lock up going forward.

My question to the Toyota gurus' is "other than the R&P, is there anything else in there that would cause it to lock up?