south Idaho thru Utah to Grand Canyon Lte March/April

I -84 to I-15 should be your best bet. You can have snow that time of year. We were at the Grand Canyon in late April one time and there was 6 inches of snow on the ground. I would make sure you carry a snow shovel, chains and Good Sam or some other type of emergency road service just in case. I bought the kind of chains that slip through your rims. They aren't made to leave on,just to help you out of a jam.