My Honda Harmony II HRZ216TDA lawn power doesn't self-propel any more

Well, my 5 year old Honda HRZ216TDA mower is sick. It has decided that the self-propelled feature no longer applies, leaving it up to me to push that heavy thing around the yard. I've posted a request for assistance from an expert at All Experts, so we'll see how that goes. Here's a list of symptoms:

1. The rear wheels no longer roll backwards (this started last year sometime)
2. The rear wheels turn when drive is engaged and the back is lifted off the ground, but stop when I set it back down.
3. On concrete, low speed 1 actually pulls the mower slowly, speed 2 barely moves it, and Hi speed 3 does nothing.
4. I removed the belt cover panels so I could access the belt and transmission. The belt seems pretty loose
5. The big spring that attaches to the Shift Arm seems to be fully compressed, and I cannot move the Shift Arm manually...at all. I sprayed a bunch of lubricant in there, but it made no difference.
6. When I engage drive (engine off, of course), the cable does move and the drive lever moves, which expands the small spring on top of the trans. This seems normal.

I don't know if it's just a bad belt, a bad spring, or if the notoriously weak transmission has finally croaked. The cable and spring are cheap, but the tranny is $110, so it's not a "test and see" type of option.

I could probably replace the spring and the belt without much trouble, but I'm concerned that the Shift Arm seems frozen against the tranny, and won't move a bit either direction. I'm kind of assuming that it's supposed to move in order to increase tension on the belt, and thus drive the wheels, but I'm not sure (I found a discussion by someone having the same problem, and it was recommended that he get the Shift Arm moving, which is what I was trying to do).

Hopefully the "expert" will come through and give me some good advice on what direction I should go. I really like my mower (those rotating front wheels are the bomb), and I'm not in a place to drop coin on a new one, so fixing this one is my best option (as long as it isn't going to cost me $200, at which point I might as well get a new one).