gear oil
It's "oil", not "fluid". Fluid is stuff you put in your windshield washer reservoir, the master cylinder reservoir, the power steering pump, and an automatic transmission. This is actually oil. That said, what kind you should use depends on what kind of differential you've got. Look in the glove box on the door, and there's a chart with lots of mystic codes that tell what kind of stuff your truck came with. One of those is for the differential. There are some examples at the end of this reply.
If you've got a positraction, a "limited-slip", differential, installed as a factory option, then the oil has to have a high enough coefficient of friction to permit the internal clutches to rub against each other with enough grip to allow the torque to be transferred to the wheel that's not spinning. You can use a wholly synthetic lubricant in the differential, as long as you also put in the friction-enhancing lubricant that comes in a tube a little larger than a toothpaste tube, and you squeeze the entire tube of goo into the differential prior to installation of the oil.
There's also a question of weight. If you're going to be driving in the middle or northern latitudes where they actually have winter, then you need a lighter viscosity, but if you're going to be drag-racing in Death Valley, then you need something that can hold up to the heat.
I have a limited slip differential that works with gears rather than clutches, so I don't need the friction additive. I use Valvoline 85-140 hypoid gear oil with about 25 percent Lucas Oil Additive. That gives me about as good a viscosity range as I can get with plenty of long-chain polymers to cushion the wear surfaces. I think that's the best-cost best-value alternative. The synthetics are great stuff, but I like to take the cover off and check around in there every eighty to hundred thousand miles or so anyway, so why not just go ahead and change the oil? Besides, it's the itty bitty pieces of abrasive metal grit that get mixed up in whatever you've got in there as a lubricant that causes the greatest amount of wear, and that's true whether you've got synthetic or genuine fossilized hydrocarbons.
G72 : AXLE REAR, 2.14 RATIO ; G75 : AXLE REAR, 3.62 RATIO ; G80 : AXLE POSITRACTION, LIMITED SLIP ; G87 : RING GEAR, 8.50 INCH ; G89 : RING GEAR, 7.50 INCH ; G92 : AXLE REAR RATIO, PERFORMANCE ; G9S : AXLE REAR RATIO, ECONOMY ; GH0 : AXLE REAR, 3.54 RATIO ; GH2 : AXLE REAR, 2.29 RATIO ; GH3 : AXLE REAR, 2.77 RATIO ; GH4 : AXLE REAR, 2.92 RATIO ; GK9 : AXLE REAR, 4.63 RATIO ; GL3 : AXLE REAR, 6.17 RATIO ; GM1 : AXLE REAR, 2.59 RATIO ; GM3 : AXLE REAR, 3.45 RATIO ; GM8 : AXLE REAR, 2.56 RATIO ; GQ1 : AXLE, STD RATIO ; GS4 : AXLE REAR, 3.70 RATIO ; GT4 : AXLE REAR, 3.73 RATIO ; GT5 : AXLE REAR, 4.10 RATIO ; GT7 : AXLE REAR, 3.33 RATIO ; GT8 : AXLE REAR, 4.10 RATIO ; GTY : AXLE, WIDE TRACK ; GU1 : AXLE REAR, 2.41 RATIO ; GU2 : AXLE REAR, 2.73 RATIO ; GU4 : AXLE REAR, 3.08 RATIO ; GU5 : AXLE REAR, 3.23 RATIO ; GU6 : AXLE REAR, 3.42 RATIO ; GV2 : AXLE REAR, 5.83 RATIO ; GV4 : AXLE REAR, 3.36 RATIO ; GV8 : AXLE REAR, 2.72 RATIO ; GW4 : AXLE REAR, 3.31 RATIO ; GW6 : AXLE REAR, 3.27 RATIO ; GW9 : AXLE REAR, 2.93 RATIO ; GX3 : RATIO,TRANSAXLE FINAL DRIVE 3.33 ; GYS : RATIO TRANSAXLE FINAL DRIVE 3.65 ; H04 : AXLE REAR, 4.11 RATIO, SINGLE SPEED ; HA3 : AXLE REAR, 5.29 RATIO, SINGLE SPEED ; HC4 : AXLE REAR, 4.56 RATIO ; HC7 : AXLE REAR, 5.13 RATIO, 7500 LBS, SINGLE SPEED ; HC8 : AXLE REAR, 3.21 RATIO, SINGLE SPEED, TRUCK ; HE3 : AXLE REAR, 3.07 RATIO, 3500 LBS, SINGLE SPEED, ; HF7 : AXLE REAR, 4.56 RATIO, IOOOO LBS, DANA 70, SINGLE ; HF8 : AXLE REAR, 4.88 RATIO ; HK1 : AXLE REAR, 2.87 RATIO ; HK9 : AXLE REAR, 5.86 RATIO 10000 LBS, SINGLE SPEED
