At one point someone recommended just disconnecting the door speakers. I had already done it at one point while I was trying out a pair of speakers I had laying around. You end up with NO mid bass at all, of course.

No and Yes.

Simply adding a sub will do nothing to stop the doors from providing sympathetic harmonics (rattling). But adding a sub and a cross-over network can stop the rattling AND improve the sound quality. This works because you can limit the low frequencies that you send to the door speakers (only frequencies between 100 and 3000Hz)

And I said "can" because as others have pointed out, if there is something loose in the door you will still need to address that.

Of course, adding a sub can also make things in the truck rattle...but that's a whole new project :)

Where can I purchase a small amount of dynamat? Not looking to cover my whole interior, just those bits pointed out in some of the posts above and see where that gets me.

ebay...amazon...Crutchfield...local car audio shop

Most companies will sell a "door pack" with enough square footage to do a pair of doors, some companies will a single sheet of their product which is typically between 1.5-2 square feet.
Most audio supply places will carry something, and dome places will sell direct to the customers. If you need some suggestions, I use Second Skin Audio products http://store.secondskinaudio.com but Raam Audio also have good products. http://www.raamaudio.com. The only product I can't recommend is Hushmat, but that is due to them taking my money but never fulfilling the products I ordered a long time ago.

If you want to improve the sound from your doors, check out my thread on sealing and deadening the doors, which is a fancy way of saying that you're turning the door into a speaker enclosure. http://www.ft86club.com/forums/showthread.php?t=28506
It's a bit more work and requires a few more materials, but makes a tremendous improvement in sound quality.