Hello. Hopefully you are still reading this board...
I have a ggg-grandfather that served in the company and regiment that you have noted above. There were very few Johnson's in this ground, maybe only 5 or 6.
William Riley Johnson, Private in Company F, 2nd Tennessee Infantry, He enlisted 12 May 1863 and was discharged 12 August 1865, serving 2 years 3 months 13 days.
If you think this could be a match for the Johnson you have, I would be happy to email you a copy of the only photo that I have of William and let you compare the two. You can contact me directly if you'd like.