FIRST, count the bolts holding the rear cover on.
10 bolts = craptastic 8.5" ring-gear axle assembly, which might have 28-spline axle shafts or it might have 30-spline axle shafts, and you'll need to know before ordering a replacement differential. The only way to know for sure is to remove an axle shaft and count/measure them. The 8.5" is BY FAR the most-common axle assembly used under GMT400 half-ton trucks.
14 bolts = either the 9.5" ring-gear axle, or the 10.5" ring-gear axle. The 9.5" is semi-floating, the 10.5" is full-floating, with the bigass hub sticking out past the wheel, the hub has 8 bolts easily visible holding the axle shaft to the hub.
Realistically, a half-ton is most-likely going to have the 8.5" ten-bolt, MAYBE it's got the 9.5" semi-float 14-bolt. It wouldn't have a 10.5 unless someone swapped it in.
There are additional axles used on GMT400s beyond the 8.5, 9.5, and 10.5, but they're rare and absolutely not available on a half-ton.
If this were me, I would not put money into an aftermarket differential for an 8.5" axle. Upgrade to the 9.5" axle, and THEN put some money into it.