I have a 75 which came with front discs and rear drums from the factory. Occasionally, the rear brakes become really touchy, and they lock up, even with light effort. This is typically the case when the car hasn't been warmed up, but sometimes even when it's warmed up it can happen.

Does anyone have any recommendations on how to start troubleshooting?

 

since I too have a 75 and unknown vehicle history, i would go through the brake system and replace a lot of components. Likely would start with new drum brake cylinders and clean up/lube all the moving parts on the drum brakes. Pads and the drums themselves might be ok, but a complete flush of brake lines would be in order too. I'm not sure what we have on our trucks to control front vs rear brake proportioning - on my 93 it has an LSPV (load sensing proportion valve) - I would assume there is something on the 75 that does this, investigate this too to make sure rears aren't grabbing too early, the fronts should be doing most of the work.

others will likely have more input, but that's where I would start investigating. good luck!