Tight Handbrake
I would pull the rear drums off and see what if anything is going on in there.
Then I would disconnect the cables from the actuators in the rear brakes and see if the cable moves when you apply the hand brake.
If the cables don't move I would assume they are frozen inside their sleeves.
As for a fix, if it is the case, see if you can get some lube or anti rust etc into the sleeve to loosen it up.