Dear Beulah,
I have been doing the process of elimination thing for a couple years on Joseph, and it has been really nerve racking. I have checked out most of the Joseph Johnson's in Tn during that time period. The 8th Tn Union I believe was raised in West Tn, and there were a couple of Joseph's in that part of the State. If I remember correctly I don't have my notes, but didn't the 8th become the 10th?
That Joseph was killed or died according to the Tn Adjutant in Feb, 1865. Which would work, but you say that one is to old
There was a Joseph Johnson who is buried at Rock Island, Illinois, the marker just say's Joseph Johnson U.S. Cavalry, not sure if he is Union or not, I have seen that for Confederate trooper's as well.

Sorry about the delay in my response, I had to go out of town unexpectedly.
Thank you so much for your help.