Well I was able to drive the tractor by holding down the clutch and starting it in 5th gear. With it running it didn't move until I released the clutch, then off I went. Once I depressed the clutch the tractor stopped. I was able to select 1st gear while slowly rolling to a stop, but I couldn't get into any other gear selection with it running and the pedal depressed. I started it with it selected in both reverse gears and 4th, but upon releasing the clutch the tractor didn't go anywhere for any of those gears. It feels like I'm in the gear, and when I crank it over the tractor wants to go in the direction for the gear selected (fwd or reverse). Put it back in 1st to put it in the barn and no problem, just can't start it in gear. I didn't try it in 2nd, 3rd, or 6th. 

I'm going to start by pulling the top of the transmission off before I split it. I have a feeling it'll be obvious what the problem is.