It was v6 auto's both trucks and runners that came stock with 31's, the door tag would read G144, if you find them at a wrecking yard still in the truck. You can buy the complete oem diffs for about $150-200 each. I would wait until you get it regeard before buying any engine upgrades, you may find you don't need them.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed