Parking brake

Better than 90% it is in the wheel brake assembly.

Sounds like the shoes are worn, or the adjusters are stuck. The shoes travel out and the E-brake levers in the rear are sticking. The bang is them releasing due to the pressure of the drum turning.

Crappy rear brakes are one of the known issues with that generation of truck.

In my decades of experience as a mechanic I found this often happens when there is a lot of brake dust in the drum, which holds moisture and "glues" the shoes to the drum. Sometimes rust is also involved. Definitely NOT an uncommon problem on GM vehicles.

Rust and the crappy design of that generations rear brakes. I've seen both ends of the spectrum in that series. Some the rear adjuster freezes and the shoes never tough the drums but they rust. At least they are easier to deal with than the later shoe in hat design of E-Brake used with the discs. Those are one step above flintstone braking..

Thanks. I see that everyone is in agreement that its in the rear brakes so that's where I'll be looking.