[vectorit]

Welcome to the forum!
The Eaton HO52 rear axle in these trucks could have been ordered with either 4.10, 4.57, and 5.14. Being that yours is a K20, chances are it is 4.57 ratio unless swapped out.

There isn't much of a market on Eaton HO-52 & 72 axle parts, let alone new ring and pinion kits.

If you are looking to go to a taller gear for better highway manners, then I would start looking for a GM full floating 14 bolt rear axle. This axle housing will have more options for gears, and the replacement parts will be much easier to source.