Technically, Governor Dan Walker doesn't count. Walker lost the Democrat Primary in 1976 and left the Governor's office after the election.
After leaving office, Dan Walker was one of the founders of the Butler Walker Auto Lube quick oil change franchise, which eventually sold out to Jiffy Lube. Walker didn't go to go to jail until 1987/1988 after committing bank fraud.
So technically Walker's an "ex governor" who went to jail after leaving office.
There are really only three Illinois Governors who went to jail for political corruption two of them after being removed from office, they are Otto Kerner, George Ryan and Rod Blagojevic.
The only reason the count isn't higher is because the rest of the corrupt sons a bitches were smart enough to not get caught.
I understand what you are saying and technically, that may be true. Technically, however, it may not. A governor like US senator retains the title for life. So ex-governor Walker was still Governor Walker when convicted and jailed.