A 3/4-ton uses an Eaton H052 rear axle -- produced from 1947-1972. '67-'72 3/4-ton trucks can be found with 4.10 pumpkins - often found behind automatics, but not always. Detroit Lockers were available, either as aftermarket or factory installs, but they went out of production around the time the axle was discontinued in '72. Bearings and seals are readily available - ask your FLAPS for parts for a 1972 3/4-ton with the H052 axle.
3.90 gears were (and maybe still are) available from an aftermarket manufacturer -- but were insanely expensive, and required a '64-'72 differential.
