Allison 1000 wont downshift on hills

[j griff]

Coming home from Carthage, MO., yesterday on I-44 - a rather hilly route - my transmission downshifted normally for the first 100 or so miles and then would not downshift, whether in cruise or not. Slowing down to come off the interstate it seemed to downshift a little slower than usual. I tried to do the "passing gear" by floorboarding it on a level stretch, but it would not downshift. I can come in and out of overdrive by using the toggle on the dash.
Is this a TCM problem ?? What might be involved in repair >> Thanks for any help.