Cump,
I "goggled" that guy and I think there is some mixup here. Posey County is in Indiana. Hardpan is now called Hardpan Ridge and it is located in northern Illinois in Ogle County. On the other hand there is a Posey, located in south central Illinois near Carlyle Illinois. That would be south of Vandalia about 40 miles or so on route 51.
I did a search for Jupiter Doke in the Illinois Data base and he don't exist. The only DOke who served with an Illinois outfit was a Sergent from Bloomington in McLean county.
The Nara database does not show anything either. I have a database of General officers from Illinois and he is not there either. So I would say he was not with the Illinois units even tho that letter says he was. Grandpa Alex was in co. F, 90th Illinois INfantry.
FC