For what Jeff is doing he doesn't need a Class A CDL
Class A License - Combination Vehicles — Any combination of vehicles with gross combination weight rating (GCWR) of 26,001 or more pounds falls in Group A, providing the GVWR of the vehicle(s) being towed is in excess of 10,000 pounds. Most Class A vehicles are trucks such as tractor-trailer or truck and trailer combinations. Driving a Class A vehicle requires considerably more skill and knowledge than driving vehicles in Classes B and C. Since these skills include those required to drive a B and C (with appropriate endorsements) vehicle, a driver who has a Class A license also may drive vehicles in Classes B and C.
Class B License - Heavy Straight Vehicles — Any single vehicle with a GVWR of 26,001 or more pounds falls in Group B, or any such vehicle towing another vehicle not in excess of 10,000 pounds GVWR. Class B includes straight trucks and large buses. Safely driving these heavy vehicles requires considerably more knowledge and skill than driving the small trucks and buses found in Class C. Since they include the skills required to drive Class C (with appropriate endorsements) vehicles, drivers who have qualified for a Class B license may also drive vehicles in Class C.
Class C License - Small Vehicles — Any single vehicle with a GVWR less than 26,001 pounds falls in Group C, or any such vehicle towing another vehicle not in excess of 10,000 pounds GVWR. However, vehicles of this size are included in the Commercial Driver License (CDL) program only if they are: 1) Designed to carry 16 or more passengers including the driver, or 2) Used to transport hazardous materials in quantities requiring placarding under the Hazardous Materials Regulations (49 CFR Part 172, Subpart F).
Here are the CDL exemptions for farm plates in SD.
Operators exempt from provisions of §§ 32-12A-1 to 32-12A-58, inclusive
32-12A-9. Operators exempt from provisions of §§ 32-12A-1 to 32-12A-58, inclusive. The following are exempt from the provisions of §§ 32-12A-1 to 32-12A-50, inclusive, 32-12A-51, and 32-12A-52 to 32-12A-58, inclusive:
(1) Operators involved in farm to market transportation movements, at least sixteen years of age holding a valid operator's license, limited to those operators of a farm vehicle:
(a) Controlled and operated by a farmer;
(b) Used to transport either agricultural products, farm machinery or farm supplies to or from a farm; and
(c) Not used in the operations of a common or contract motor carrier;
(2) Operators of emergency fire fighting equipment necessary to the preservation of life or property or the execution of emergency governmental functions performed under emergency conditions that are not subject to normal traffic regulation, or nonemergency conditions when operated by members of a fire fighting agency;
(3) Operators of military vehicles for military purposes including:
(a) Active duty military personnel;
(b) Members of the military reserves;
(c) Members of the national guard on active duty, including personnel on full-time national guard duty, personnel on part-time national guard training and national guard military technicians (civilians who are required to wear military uniforms); and
(d) Active duty U.S. Coast Guard personnel;
(4) Operators of recreational vehicles; and
(5) Operators of rental transporting equipment used as personal family use vehicles.
United States reserve technicians are not exempt under the provisions of subdivision (3) of this section.
Source: SL 1989, ch 267, § 7; SL 1995, ch 180; SDCL 32-12-85; SL 2001, ch 171, §§ 79, 115; SL 2004, ch 213, § 2.
The SD DOT website has all the information. Best bet is to call them.