i could reccomend you one ten thats plenty enough for a truck. Try the Orion HCCA series subwoofers. extremely strong and reliable for the price.
there are plenty of strong single 12" subs out there. it depends on how loud you want your system. You planning to give your 12 sub 500 watts rms , 1000 watts rms , 1500, 2000 ??
500 should be enough if you're weak. 1000 is getting kind of lound. 1500+ is really loud.
When you buy an amp or subwoofer make sure you look for the rms rating and not the max. Get a single channel monoblock amp.
oh yea, if you plan on giving your truck more than 1000 watts rms, make sure to get an extra battery and cap, so your system doesnt dim your lights (eventually killing your alternator in the future). If you're throwing 1500+ watts rms in your truck, you should just get a high output alternator.