Welcome to the forum! Your GW would come with either York or Sanden compressor - probably Sanden, but I'm not sure where they made the switch. My 88 has York. I'd go with Sanden as it's better/newer design and more efficient than York.
Here is a picture from parts catalog showing both York and Sanden brackets, which one is looking like what you got?
I wouldn't bother with R-12 refrigerant and go straight to R134a. There are kits available that have everything required for conversion (in your case it's not really a conversion
, since you don't have anything AC-related under the hood), here is a link for one on eBay
that I was going to buy if I had to convert to R134a. Luckily my system only needed new resistor, blower motor and a bit of R-12, but I saved the link just in case
In general, for identifying parts and overall education about your rig I suggest getting factory TSM (service manual) and parts catalog. Don't bother with Chiltons or Haynes books, TSM is better. For your year both are available on oljeep.com
site in PDF format - 1989 TSM
(careful, it's 345 Mb) and parts catalog
(35 Mb). You can also get paper version of TSM from BJ's Offroad