Problem solved. Thank you Eugene Rounds. The weld had indeed failed at one end of the rod and the local mechanic was able to re-weld the rod into the socket and get the mower good as new for 1/2 hour's labor time -- roughly the same cost as a new part if I'd tried to replace the bar myself.
It's all there. It's conceivable that a weld failed at the end of part 35, giving it enough play to deflect instead of rotating the opposite end of the bar. It does not look that way to me, because even when the bar is seated in the hole on the end plate, the bar goes diagonal rather than transferring torque to the opposite wheel assembly. Thanks for your help. I'd welcome any further thoughts from you or others, otherwise I guess it's a trip to the shop for this one. It's been a great performing machine, taken a lot of abuse on rough meadows, and aside from this it's always been easy to diagnose and repair.
Thanks. It the part appears to have been inserted securely into the holes on the brackets that fit inside the wheels on either end of the axle. But even with the rod in place, when i raise the right rear wheel using the attached lever, the bar deflects and moves to a diagonal position so that the left rear wheel is not raised. The bar that is intended to transfer the position change to the front wheels does not move, because the motion of the lever has been accomodated by the diagonal shift of the back bar, shown, rather than by an upward movement of the other back wheel. Is it possible that there was a part that held that shaft in place so that it could not deflect and would have needed to move the other rear wheel?
Hello I got the model number wrong. It is 7021-R. The product number is 96143006300. I've uploaded a photo of the bar that fits into the wheel mounts.