I did Tennessee because I was expecting for it to be a tad more interesting than what it turned out to be. There were a number of German and Swiss-German colonies built along the Cumberland Plateau between 1840 and 1890, but several failed and the residents ultimately migrated to Knoxville (which already had a fairly large Swiss-German community at the time) or dispersed. I wasn't able to track down all of them, though, but there are still quite a few notable cities remaining (Hohenwald, Gruetli-Laager, Wartburg).
Potentially interesting finds: OK, so after a bit more digging, I tried to see if any of these counties with larger German populations might actually have a significant Catholic population. The only example I could find that really stood out was Williamson County, which isn't a rural county anymore (pop. 200,000).
I still think this is quite interesting, as there is no other urban or metro county in Tennessee that has anywhere near the Catholic population (21%) that it does. It's also tied for a close third in German population percentage. What is odd to me about its Catholic population is that usually, you don't find large populations of Catholics in the South in a suburban county without there being a comparably-sized or larger Catholic population in an adjacent metro area. Nashville/Davidson has 9%; Memphis/Shelby has 14%; Knoxville/Knox and Chattanooga/Hamilton each have 6%. So I'm pondering whether this county - before the most recent metro explosion in growth occurred over the past 30 years - had a much larger rural German population that was indeed Catholic.
I did find some more interesting auxiliary connections in the area: Maury County (adjacent; to the South) also has a larger than normal Catholic population (11%) and is adjacent to two other counties (Lewis, Lawrence) that once had extensive German "colony" projects in them:
Lewis, which is home to Hohenwald:
County: German % Catholic %
Morgan: 24% 1%
Cumberland: 15% 8%
Loudon: 13% 2%
Williamson: 13% 21%
So this general area seems to have some potential for German Catholic rural consideration:
I haven't checked to see if there is a perfect correlation, but this also seems to be in the same general vicinity as the part of TN that has always been stubbornly Republican...