you technically could swap the drums for rotors but it would be too much work. it would have been easier if there was a higher model with discs so you can just swap parts. i have researched and i have found that you need a way to mount the disc calipers to the axle. on old beetles there are mounting brackets with tabs so you can bolt the calipers to. however i cannot find one for the first gen trucks. your best bet would be to either get rear discs and modify the mounting points and lug pattern so they fit; or make your own mounting brackets. even then you have to think about the brake proportioning the brake lines, the parking brake connections as well as fitment to the rims you have and the anti lock braking control. maybe you can just upgrade what you have to compensate for the rear drums? better braking fluid, bigger booster, lightweight rims, better tires, good pads and front rotors and upgraded brake lines?
the "bracket" i am talking about for vw's it the silver ones on the top left you can see where the 4 holes attach to the axle and the 2 on the flange attach to the calipers...
good luck and keep us posted.