Best way to really find out where the noise is coming from is to pull both drums out and take a peak. Actually with the drums on, you could have someone inside the truck put it in drive and let the rear axle move by its own (with the rear jacked up obviously, and the front chalked) and you could probably track down the noise from there (if it happens). When putting it into park, pull the e-brake up so you don't shift it while it's moving.

I did this just to see what the f*** would happen when I did my bell crank replacements a while ago but this might be a good way to track down the small noise.

While you are there with the drums, spray the whole area with brake cleaner and see if the noise goes away. It might work. Keep in mind if they haven't been off in years, you might want to back off the adjuster behind the drum shield and pound the drum with a rubber mallet.