@brian I am going to kiss you if we ever meet!
I have been having this exact same issue with my 79 fj40. I have new OEM drums with SOR shoes. I have been having the same squealed when shoes make contact with the drum. The exact same issue.
I still have a lingering issue with the rear brake squeak.
Starts when the pad comes in contact with the inside of the drum. I cannot get it to squeak squeak when breaking in reverse and goes away if a slam on the breaks hard enough. I have been telling myself the pads just needs to ''seat''...
Honest option is that this 40 has rust in the rear corner panels, above the rear wheel wells and probably in the rocker panel, front driver fender has been rolled in. Almost every 40 series has rust in one place or another. Rust alone should not be a deal breaker. Bondo is not the long term...