Well, I didn't see anyplace to put the special cotter pin in the tiller shaft. I thought I looked it over pretty good. I can see on the shaft for the mower/snowblower where the pin would be installed, but don't see anywhere on the tiller. I guess it could be broken off.
What does a special cotter pin look like? I saw it referenced in the parts manual, but haven't seen one.
The freewheeling pin was pulled, and also frozen so I was trying to the the tiller off so I could get to it. Then when I couldn't get the shaft off the PTO, I just worked a hammer in and managed to drive the pin in. Of course, now I can't pull it out, but I sluiced it good with Kroil, so hopefully when I get back home I can get it moving.
Yours may not have the Hole in the shaft for the pin (Some had them and some didnt)
If the rear pins and lift bracket were taken off the tiller should just slide right out.
My Next guess judging by the tractor, it was left outside and the shaft may be rusted in place.
I'd spray some kroil near it and see if it breaks free, you may even want to run the tractor and turn the pto on and off a few times as that may Loosen its grip on the splines.