I have a 1979 Cadillac deville, and would like to switch rear end gears. Currently it has 2.2:1 gear. 18/41 and would like to find a 4:10. This is for an off road application.

details I can provide, it has a ten bolt cover that has the ) ( look on the side of the cover. (I thought this meant 8.2”) it has 28 spline axles, it has 1-1/4” pinion nut.
but the confusing bit for me is it has a 12 mounting bolt open carrier and ring gear and to the best my eyes can do the ring gear itself measures 9” outer most point of gear directly across.
the bolts holding the ring gear on were 3/4” head reverse thread.

I having a very difficult time locating a gear to replace my current setup

I've never heard of a 2.2:1 differential ratio, 2.41:1 is the tallest that I've heard of. Could you re-count and see if you have 17/41 instead of 18/41 ?

These rear axles aren't known for having a lot of strength. If you are looking to go to tall tires it probably won't put up with it for long.

Can you find the build code stamped into it?