Rear brakes are pulsing when warm - why?

[jdub]

Either the left, right, or both rear brakes on my '87 4wdGL wagon are pulsing when the car is driven for a time (presumably they are getting warmed up because this doesn't happen when car is "cold").

 

Any idea what this might be? Clearance problem? I have done no work in that area nor have I bled the brakes - this just came outta nowhere. The only consistent fact is that the car hadn't been driven in some time, so I wonder about internal corrosion on the drum surface from water ingress?

 

Many thanks!

jw