If you just have to keep the factory headunit
an inexpensive route would be an LOC and bass blockers on the front speakers to block out the lows they really can't play well anyway (and will now be played by the sub). If the regular cab has the plug installed for rear speakers install the LOC to that output and you will be able to adjust level between the sub and front speakers "on the fly". If there is no rear output on the regular cab radio (I'm pretty sure there is but it may not have the plug installed) an in-line level control on the sub will do the same thing. In any case you will still be limited by the bass limiter in the headunit but from what you have stated it appears that may not be a problem for you.
Now on the other hand....
Although I'm sure you already have the idea that an aftermarket headunit would be the ideal way to go if possible
I still have to mention that IMO even an inexpensive $99.00 installed headunit special from Circuit City/Best Buy would be a better/easier solution as mentioned.