Low-Tech-Redneck

PA requirments for a CDL is a vehicle that

A. Weighs 26,001 pounds or more
B. Tows a trailer that weighs in excess of 10,000
C. Carries 15 or more people, excluding the driver
D. Carries placarded cargo

That's the basic requirments, though I'm sure there are loopholes for historical vehicles or vehicles that are classified by the DMV as "Implements of husbandry or agriculture" (farm trucks) or construction equipment.

Curiously, nowhere in the regulations is it mentioned that one needs a CDL for air brake vehicles (Though you need to be able to demonstrate how to use an air brake to pass the CDL test) there is also no mention of number of powered axles, or total axles for that matter.

As always, check the law before you have to defend yourself from someone with a ticket book and too much free time