Most shops would probably tell you you can't just recharge AC, if it's blowing warm that means the refrigerant has leaked out somewhere, and a part needs to be replaced, since it's a sealed system and the refrigerant doesn't get used up. It'd suck to have to do it twice, and pay twice for the refrigerant.
At least in CA, it's not legal to buy R12 unless you have a license to work on these systems, and the fittings need to be changed, along with some other pieces, depending on the vehicle. I haven't done this on a Cherokee, but I'm just speaking from my own experience from when I worked at a shop.
When a customer would bring their car in, we would recharge it. If we couldn't find the problem right away (faulty AC compressor, etc). We would also put in some leak dye, and send the customers on their way, telling them to keep an eye on it and bring it back when it started to get warm again, knowing that we first needed to find the leak/malfunction before fixing the problem. Then, when they came back, we'd be able to trace down the leak, and repair the defect. We wouldn't charge them a second time for refrigerant, which is nice because it's getting expensive!
I know this isn't helpful from a do-it-yourself perspective, but maybe it helps to know a bit about how the mechanics would handle it.
03 WJ Limited
, 4" Zone, 265/75/16 Duratracs, Addco rear swaybar
89 XJ Pionee
r: Stock with 31s and a trim. Sold
99 XJ Sport
: 2" on 31s with armor and dents. Sold
74 Nova - 2" drop and disc conv. Sold.