Do you have to have a CDL to drive a single axle truck in Montana?

It all depends. Single axle trucks.. tend to be a bit ambiguous. For example, the Ford F750 is sold as a 26,000 GVWR truck, even though it has 31,000 lb. axles. So long as that vehicle remains at 26,000 lbs. or less, the weight rating isn't enough to warrant a CDL. If that vehicles grosses out at more than 26,000 lbs., then it requires a CDL.

Additionally, if the truck itself is rated at only 26,000 lbs, but you're towing a trailer, then you've created a combination over 26,000 GVWR, which requires a CDL - Class B if the trailer is rated at less than 10,000 lbs, Class A is the trailer is rated over 10,000 lbs.

Additionally, if that vehicle is carrying a quantity of hazardous material which requires placarding, or is designed to transport more than 15 passengers (including the driver), then a CDL with the appropriate endorsements is required to operate it.