Just checking for sure!

So the time has come once again for me to get my rear end for my 51 plymouth. It's been awhile since I have talked or even thought about getting this but I have been told in the past that a mid-90's jeep cherokee will work fine but now that I am actually going to get it this week I just want to make certain!...does it matter if its 4x2 vs. 4x4?...and how do I tell if I pick it myself what gear ratio it is. I have been told to go with the 3:55 or 3:73 but like I said if I am picking it myself how do I know?...thanks guys...

 

I'm using a rear from a mid '80s 5th Avenue in my '53 Plymouth...