how difficult on a 91 mazda b2000 to change the rear oil seal
Assume you mean the rear main seal on the motor? Most of the work is removing&reinstalling the drive shaft, the transmission, the flywheel (or flex plate if its an automatic), then you can get at the seal. I'd allow a day for this project for an adverturesome shade-tree guy. A good Mazda guy could do it in 5 hrs. A transmission jack is handy but you can get by with a regular floor jack. It is sort of a 2-man project if you're working on a garage floor without shop type equipment.