Rear Brakes locking up

It's a good idea to go around all four corners to check and readjust; question I have for you is; have you just done a bunch of brake work?? Master cylinder, wheel cylinders? If so, is the master cylinder really a drum brake MC, and what size are the wheel cylinders? You're dealing with hydraulics, force over area, if the wheel cylinders are a larger diameter than original........even though parts book says it's the right one.....the larger surface area of the piston will generate more force on the shoes and cause lock up.

It's a long shot to be sure, if you haven't done any brake work forget what I said; if you have it gives you something to think about.

Good luck, be safe.