80W90, but really anything that is GL5 gear oil is fine.
I have Sta-Lube 80W90 in the front and rear diffs on my '94 Ranger because it came in a convenient gallon jug. My '02 has synthetic Valvoline 75W90 in it. Ford specified 75W140 for many 8.8s, and I have used that too. I'm going to change the front & rear diff fluid in my '94 Explorer soon and will use whatever 80W90 I find in a gallon container...Coastal, Sta-Lube, etc.
If the truck has a limited slip diff, you need to make sure to use friction modified fluid or buy an additive. You can find out if it is limited slip from the axle code on the door jamb.