It is very easy to spend a month anywhere..
Yep. That I would grant!
unfortunately we will have to make do with two weeks... The big day is a wedding so we want to try and make the most of it and see bits that we might not have another chance to see..
OK. First night I would stay in St. George, Utah. Drive through Zion National Park the next morning and head north on US-Hwy 89. Hwy 89 has great views and moves nearly as quickly as I-15. (It is slower because of the cities and towns, but you will be OK timewise). Night #2 could be near Salt Lake City. The tour at Temple Square is worth seeing no matter what you might think of the the Mormon faith. From here, I don't know where in Montana you are headed, but I would probably suggest you push up I-15 to I-90 to your final destination. You may need to drop Temple Square and push on towards Pocatello, Idaho to make that third day more relaxing.

Return trip: Let us know where the wedding is and we can share tips for the route back to LA.

Mark