If we can tolerate some geographical diversity, the Texas(I think) band called The Reivers (briefly known as Zeitgeist) also deserves mention among the pioneers of Alt. Country. I believe that they predated Uncle Tupelo, but I'm not certain about that. Their particular variation included more vocal harmony and - to the extent it received any notice at all - was sometimes called the "New Sincerity" movement. Any of their records is worth owning if this is the tree you want to bark up.
Marty
Marty