When you take off the drum, look at everything for any clues. Uneven contact, drum slightly out of round, any sign of where it's grabbing.

There's some good brake tech from Chrysler in the MTSC pamphlets.
The Imperial Club has almost all of them on line.
go to www.imperialclub.org -> Literature -> Master Technicians Conference
Scroll down to the 1960s and 70s for drum brakes closest to your year.
1969 is the first year for the new revised self-adjuster hardware and shoes.