So after doing a fair bit of research on the best subs for a Wrangler as well as something that wouldn't break the bank, I finally went with this:
Boss Low Profile Subwoofer - Walmart.com
(It hurt a bit to give Walmart business... but, ya know... it was cheap)
I just finished wiring it all up today and 3 hours later I can honestly say it was well worth the small amount of money I paid. If you're looking for the kinda bass that'll shear your vinyl windows from their zippers, this isn't for you, but the quality of the sound is astonishingly good. With a bit of EQ tweaking I managed to get my soundstage set up as well as I possibly can in a Wrangler. It's a night and day difference, fills out the sound so well. Also comes with a cute little Bass Level knob that I mounted under the dash.
And it's attached with velcro strips, and I've got every cable bundled together near the inputs, so if I ever need to remove it, it's a 1 min job.
Overall, very satisfied with this thing. And just to mention I've upgraded all my other speakers to Alpine and I'm running a Pioneer deh-x8500bs.