I ran into this also on my 91. The issue for me was the shoes had actually bonded themselves to the drum. I tried backing the adjuster all the way in and pulling the wheel cylinder bleeder out; none of these worked. Worse, as I tried to force it off the shoes both bolt holes in the drum stripped, giving me no means of leverage.
My final solution was to disconnect the parking brake cable at the adjuster and all mount screws for the cable, remove the brake line going into the wheel cylinder, unbolt the four nuts at the back of the backing plate (the axle mounting nuts) and pull the drum and axle out as an assembly. I drilled another hole and used a self-tapping screw (about the same size as needed for the factory holes) and the drum came right off.
I just had this same problem. But I can't take credit, I found on the web a suggestion that worked for me. Along with the other answers, you make sure the parking break is off. On the drum you should see 2 small screw holes some what close to the center. Find 2 bolts that will fit and gently tighten each one in. They will "push" the drum off of the inner thing (I'm forgetting the name), and sure enough, each of my drums came right off. I "borrowed" 1 bolt from the each tailgate latch, they fit just right. I think it was a 12mm socket. (Its been a long day and I'm not an expert on part names. by "tailgate latch" I mean the metal brackets on either side of the bed gate that the tailgate closes into. Should be 2 bolts on each side.) i hope that helps. Now for what brought me here... the E-brake doesn't seem to work the way it should.
Get a 2x4 about 6 inches long and a heavy hammer preferably one at least 2 pounds.First make sure your parking brakes are disengaged by hand pushing the top of the P arking brake cam in on the backing plate,now if your drums are rusted in place like mine were put that hammer in back on the back rim of the drums with that 2x4 and hammer and at the same time go around the back of the drum edge hammering the 2x4. The drum will come off,trust me no ones d21 wasn't worse than mine.I bought a 1990 d21 4x4 that sat for a year and my drums only came off with brute hammering if you bust them not to wory there not that much money.