Wow, that's quite a response curve you got there! Have you also corrected the readings to compensate for the SPL meter mic's inaccuracies? Here's a couple of links that give you the correction values for the Radio Shack SPL meter.

http://www.svsubwoofers.com/faq_rscomp.htm
http://www.audioc.com/library1/testcd/testcd.htm

For your setup, Terrence is right in that repositioning is probably where you should start. The rule of thumb that's worked for me is to setup the subwoofer at the seating position and play a test tone (or even music) through it. Go low to the ground and crawl around the room until you locate the spot where the bass seems fullest and most even. Once you reposition it, then remeasure it, and do the necessary corrections with the BFD. Extremes as big as the ones in your room are very common. The BFD will help with the peaks, but it won't do much for the cancellations, so try to find a location for the sub that minimizes the dips.