That obvious diversion as far North as Missoula seems prime for pruning to me.
I'm planning on doing a cut-off in Idaho on a West to East starting in June. While my planned route probably won't save time (LOTS of climbing via Lowell, Stanley, Challis, then Rexburg & Teton Pass) - it seems to me you could divert from Jackson - over Teton Pass to Rexburg, then to Arco, then follow US 20 to the Boise area. Then you'd just pick up US 26 in Ontario, OR
Not sure the best way to noodle from the where US 20 hits I-84, but I seem to remember someone mentioning in a previous thread here a way to do it on non-interstate roads via Mountain Home