Sorry this is almost a year late! Just stumbled on the thread. Did you solve the "mule" problem?
I have a 1982 GTV 11, most of which (except for the engine) I think is the pretty much the same as the GTV 16 and GTV 18. The mower deck is model 917.253644, 44 inches. The assembly I think you want is called "idler box" in the manual.
Let me know if you still need info--I can scan the parts drawings and list for you.