You are going to want something that is used for small spaces such as the Kicker Solobaric or other similar subs. When looking at subs, look at the required enclosure space. You'd probably be around 1 cu ft under the seat. Be sure to get a sub to match.

 

My personal favorite is Rockford Fosgate.
Check out Crutchfield.com. they list all info you will ever need in choosing a sub.

got my box that fits under the passenger seat which is the exact cubic feet required for my sub online at SOund Off Audio out of Florida. found these guys through extensive web searches

 

Two things.....

First: You don't need a huge speaker/system to make a lot of noise/bass.....I've seen ONE 15" subwoofer beat out several 2-3 speaker setups.....

Second: There are several good speakers that would work really well in your truck......Kicker comp and Rockford Fosgates are pretty dang good.....not to mention....cheaper

For the record......it's not always the brand name stuff that works out the best....what I am saying is that you should focus more on getting a solid setup rather then worrying about name brand stuff.....you're wasting your money paying for top notch stuff if your install/setup doesnt match.....

For your truck....i would go with 2 10" Kicker Solobaric S10L7's and a 1200-1500 watt amp.....

Mounting depth is 6 3/8 in.....requires min of .66 cubic in enclosure...... THESE WILL FIT UNDER YOUR REAR SEAT.....they fit in my 01 Dakota.....and they were under my rear fold down seat....