First, sounds to me like the front 19.5's are the correct wheels, that's for 4000 series trucks, even today. Wanna sell them. Those tires can be had most places, just might be $250 or some less each! They used them on Motorhomes alot. Second, sounds like you have Budds on back? Big 1-1/2" nuts to hold outer wheel, then the square headed Budd nut to hold inner, one side is left handed. Snap-on makes sockets, guess you bought that one for both types. Oil and long bar on supported wrench at wheel is the right answer most times. As for the drums, take off with hub and all. Remove the axle or axle cover on outside end of axle, may be 8 to 10 bolts, some have tapered wedges too. If tapered wedges are there, loosen nuts alot, don't remove. Then hit on axle end with 2 to 4 pound hammer a good "Rap" and the tapers should loosen and then remove nuts and tapers. Pull the axle out, it will be oily and stinky, set up on end aside somewhere. In the hub will be two big nuts and a lock ring between them. The ring may have tabs bent over into slots on the outer nut. Bend them back, buy those big tin sockets in size you need to fit your nut. Loosen outer nut, remove the lock ring, the inner nut should arleady be loose or nearly so. when nuts are off, whole hub, bearings and all will come off as one. The outer bearing may come out easy, don't drop in dirt. With outer bearing out, the hub may move around more and be easier to "work" off. If the brakes aren't hanging up on them, they should come off. Maybe gentle blows with big hammer around edges to assist. Get new seals for hub in back, repack bearings some. You may not even have to clean and pack, if well lubed now, just keep them clean, and don't mix sides. Most of these rears have two cylinders per wheel and can get expensive, plus hard to find. Be prepared to open wallet and wait! I once had a Ford F-600 truck, with that size tires, said "Made for U-haul" on them, not for resale. If this is a changed axle, bet it's a Ford or Eaton type, not GM at all. Good Luck!
