I have a thread that mentions fitting a subwoofer in a standard cab ranger.
You have quite a few choices actually. You can either get an entry level subwoofer with a shallow mounting depth, or get a full blown shallow mount subwoofer.
Kicker, Rockford Fosgate(which is a great brand btw, reguardless of what people in this thread may say), and Pioneer are probably the best offerings.
JL makes a shallow mount sub now I believe, but I'm sure it's not the best bang for buck buy out there.
I think of all the shallow mount subs pre-enclosed rockford fosgate probably makes the best offering. The surround is very thick and it looks like it could put out some good excursion.
The kicker is probably gonna be what I go with though.
Some people say get 2 10inch subs, well it depends on what type of bass you like. 10inch subs generally have tighter bass, but if you go up in size the bass tends to get a lot deeper.
12inch subwoofers are my personal favorite, because they tend to have deep bass while still maintaining the quality of a 10inch sub, so keep that in mind.