Any suggestions on a sound system for a jeep wrangler?

You can look around for some more waterproof speakers that can take some light rain, but that's about it. I think in your case a two speaker system will be best, less wiring and if you get coax speakers it'll be good stereo sound. Most rear speakers don't have much volume going through them anyway and in the end, won't be worth the wiring/installation headache. I've had good luck with Alpine decks and speakers for their various ranges and impedance depending on what you want. There are lower cost systems available and you might luck out and find one used.

Check crutchfield.com they can fix you right up. But I would not spend much, the road and wind noise is going to drown out everything else.