I just changed the differential lube in my Ranger and the owners manual calls out for 75W-90. I have a Torsen in my rear differential and the tag on it states to use only 75W-140 synthetic but that is only for the Torsen. I used 75W-90 in the front and if you have the factory Trak-Lok limited slip differential in the rear you should use the friction additive with the 75W-90 gear lube. The synthetic I used said is good for standard and limited slip differentials but I would still use the friction modifier additive with a clutch type limited slip differential.
2002 FX4 Bright Red Supercab 4.0 SOHC, manual 5-spd & Tcase, 33" BFG MTs, K&N air filter, Modified air box inlet, Gibson SS cat. back exhaust, Bama flash, Hidden Hitch round receiver, Rear helper air springs, Electric trailer brake controller, Line-X bed liner, WAAG bar with KC lights and Wildernest shell.