From GTNP go south to Jackson, then follow US 191/26/ to Hoback Junction and continue on US 89. At Geneva, ID you can follow US 89 or continue S. on ID 61/WY 89 to Evanston, WY at I-80. I'd get off I-80 at Exit 146 to take US 189 S to bypass Salt Lake City. It meets up with I-15 in Orem, UT.
Alternate Route: At Geneva, ID follow US 89 past Bear Lake to Bringham City. US 91 is a short jog to I-15, bypassing I-80 completely.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI