[MJN]

Assuming it is a 1/2ton they originally they came with a GM Eaton-type HO33 axle with either 3.90 or 4.11 gears in them. I don't believe there are aftermarket gears available for them any more.
The options for different ratios are sort of limited to a different rear axle such as a 12 bolt and relocating the spring perches or adding an OD transmission or auxiliary OD unit.

The 3/4 and 1 tons came with GM Eaton-type HO52 and 72 axles with 4.56 or 5.14 gears. 4.56's were not available in the 1 tons. There are no aftermarket gears for these axles and would again require replacing with a GM 14 bolt (or similar) and relocating the spring perches and shock mounts. You could replace the third member with one with 4.11's but that is the best you'll find for the Eaton-type axle.