How about a Focal 5WS 5-3/8" subwoofer, Car Audio mag compared it with a 8" sub and he siad it actually went lower than the 8" did. 80W power hadling, tho. no good really unless you use it with a component system.
This Visaton 6.5" driver has a super long excursion (22mm) and that would definiety sweep more air than your average 8" sub! The biggest sealed enclosure it needs is 0.35ft3 but can only handle 70WRMS.
I have speakers made with these in them and they actually produce a lot of bass. almost as much as my LA P.3.10 sub that was in my mustang that now resides next to my comp desk. But they timbre perfectly. Also, this one can only handle 75WRMS. The paper cone sounds awesome!
Here's a 6.5" subwoofer with a 11.5mm excursion! that's almost 1/2" from a 6.5" driver! Anyways, the F3 (-3db response point) is 70hz in a 0.15ft3 which is pretty good. I'm not so sure about how it will sound in the ranger cabin, but if the enclosure were any bigger, it would definitely lower that f3. And it's only 40 bucks, so if it sounds like dookie, just pull it out.
Sound splinter makes really good sounding subs. They're 8" is really freakin massive!! it can handle serious power and uh, would probably have to be mounted backwards...
That's pretty much all I could think of...