To not answer your question-
Just buy the backing plates new from rock auto for under $12 a pop.
To answer your question-
The axle to caliper brackets are different between 2wd and 4wd. Calipers are the same, rotors on a 4wd have like 1" wider of "hat" where the e brake drum sits. Otherwise its pretty similar.
4wd puts the calipers forward of the axle centerline, 2wd puts the calipers rear of axle centerline. The calipers on a G body forward of axle centerline will hit the frame.
A right 2wd backing plate will be very similar to a left 4wd backing plate, but the caliper is clocked a few degrees in a different direction between the two. (see photo below)
A 2wd backing plate can be used on a 4wd axle and vice versa, you just need a tin snips and a few mins, also you have to be OK with another 1 inch of rotor showing or something but since nobody is mud bogging their G body its nothing to worry about.
If you are a real tightwad you can buy an entire 7.5 disk brake 4wd setup, flip the brackets so the calipers go on the opposite side from the blazer, buy new rotors, and reuse everything else. You just loose E brake capability unless you build another bracket.
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