I don't think that in Michigan you have to have a CDL endorsement if you are (a) a farmer, (b) a mechanic. And, as it's been said, the line is at 26,000 lbs.
Either that, or all of the farmers and mechanics that I know are driving large trucks without the proper paperwork.
School Bus Drivers have a CDL Class C or D (Can't Remember which), and the Linemen at the Electric company have a minimum of a CDL Class A.
My 46 is 3400 lbs (at least, that's what it says on the title... I've not actually verified it) so I don't have much to worry about.
