It's probably a GM corporate 14 bolt. You can identify it by the "drop out" style pinion support group, 14 bolts on the diff cover, and large lip hanging down. A 14 bolt is roughly equal to a Dana 70 in strength with a 10. 5" ring gear and 1. 5" 30 spline axles. The 14 bolt is a very desireable axle in my opinion. It's very easy to work on, very strong, and has good aftermarket following and parts availability.
The factory "posi" offering in a 14 bolt is a Gov-Loc. A Gov-Loc uses plate clutches to provide a small amount of limited slip style torque transfer. It also has a centrifugal mechanism that senses excessive speed differential between the two sides and will lock 100%. With anything larger than stock tires, this locking action has earned Gov-Locs the nickname "Gov-bomb".
I'm thinking that a '90 model would have drum brakes, but it's really easy and not very costly to convert to rear discs using mostly junkyard or parts store parts.
The truck could have a Dana 70 rear, but that's not as common. If it's the Dana, it will probably have "70" cast into the differential housing. It will have a longer, non removeable pinion snout. The diff cover will be the same size and shape as a Dana 60 and may also have 14 bolts, but I'm not sure about that. The Dana 70 also has a 10. 5" ring gear, but there are several options for the axle shafts. Most common is 1. 5" 35 spline. Less common is 33 spline and 37 spline axles.
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