I'm not familiar with the axles they put under the K20's, but if they are the Eaton axles you aren't going to have many choices for gears. The good news is that GM used these axles for a long time, but the only factory gearing I am aware of is 4.56 and 4.10 and maybe a 5.14 or something along those lines. There is/was an aftermarket 3.90 set available that depending on the year of the third member is either a bolt-in or needs some minor modification. There are previous threads about this on this site, and I know there are a few on here that have swapped and used the aftermarket gears, so maybe they will chime in.
I also believe that Dana axles were an option on certain trucks from this era. I've never seen one myself, but if you have one of these, it seems like it might be easier to get different gears.
Aaron
