So you basically have 37"x25"x20" (you'll want to look at it in terms of inches as that is how most box measurements are advertised). Sounds like you have tons of room. Now in a Jeep, is it a Wrangler where the box is gonna be out in the open or like a Cherokee/SUV type? What kind of music do you listen to? You have enough room to basically put whatever you want back there (within reason) so it all depends on what you like. If you primarily listen to rap, you'll probably want to go with 12s as they will give you deeper bass (generally) especially if you are using a prefabbed box. Designing your own, you could pull deep bass out of 10s, but that is another story. If you like rock/metal, go with 10s more than likely. You have room enough for two of either of those in either a ported or sealed box.
Now most boxes aren't gonna be 25" deep. You'll probably see pretty common sizes for 2 subs are gonna be around 30" to 36" wide (which you have), anywhere from 12" to 16" deep, and 12" to 16" tall. After you pick your subs (if you are going with a prefabbed box), check what the manufacturer says is the 'recommended airspace' for whatever kind of box you go with (I'd do sealed as they are more forgiving when it comes to not being designed for a specific sub). Next, find a box that fits within that range and you should be good to go.
Now if you are looking for 10s or 12s, last years' Alpine Type Rs can be had for as low as $200 shipped for 2 on
ebay. Do a search for SWR-1242D or SWR-1042D. You'll also need to figure out if your amp is bridgeable (if 2 channel) or mono, and what it is stable at (2 ohms, 4 ohms, etc). If it is a mono amp that is stable at 2 ohms for example, then you will want to get 2 Dual 2 ohm subs (the Alpines are Dual Voice Coil), and wire them in series parallel (i think). There are very simple wiring diagrams that come with the sub, or that can be found on here. If it is a 2 channel amp that you are going to bridge, it is more than likely stable at 4 ohms bridged, in which case you will want to do 2 Dual 4 ohm subs. Now the Type Rs will probably want more power than 600 watts shared, so you may want to either consider a bigger amp or get different subs- I was just using those as an example for your price range. Good luck- hopefully that helps!
Mike