Crossing the Rocky Mt.s in Montana ??

Final destination .... Bay City MI !!

Done both. But depends on your final destination.
Last summer came east on US-2. And then dropped down to I-90. Beautiful drive through Montana.

No high cliffs on either. And the grades are no worse than on I90 in the other direction.

US2 crosses the Rockies south of Glacier via one of the lowest pass.

I've taken US 2 and it was not bad at all. Lowest pass through the Rockies in NA. Steeper grades on the Interstate in Pennsylvania.

I've been all the way on Hwy 2 and don't recall any really steep grades. You won't have cell phone service except in towns. That's in Montana from eastern Glacier going south. But going from Kalispell you have to hit some grades tho I haven't been on that section. Marias pass is on that route.

If Shelby is your destination Hwy 2 is definitely the way to go-no steep cliffs either way but lots more ups and downs on the interstate. BUT--watch for deer and sometimes elk on the hwy between Bonners Ferry, Id and Kalispell, Mt.!!!!!!

You don't have any options then. There is no route without steep hills, etc.

That being said US-2 probably has fewer then I-90.