The only system I know is the TMTC one here in Texas - this is from their website:

OHV Trail Ratings:

¨Level 1 (May need a note on some regarding WIDE/Narrow or Full-Size)
Technical Recommendations: 4-wheel drive only needed occasionally. Relaxing ride with few tough spots.
Description: Any 4-wheel drive Sport Utility or Pickup with a low range and reasonable ground clearance can do these trails. Obstacles may include some deep sand, shallow water crossings, small to medium rocks, mud, and semi-steep hills and descents.

¨¨Level 2 (May need a note on some regarding WIDE/Narrow or Full-Size)
Technical Recommendations: 4-wheel drive needed a lot. Stock vehicles and tires OK.
Description: An SUV or 4-wheel drive with larger (than) stock tires, possibly a limited slip, and a transfer case ratio of 2.5 or above fares better here. Some SUV's will be seriously challenged. 50/50 locked transfer case recommended.

¨¨¨Level 3 (May need a note on some regarding WIDE/Narrow or Full-Size)
Technical Recommendations: Large, aggressive tires needed. Rear locker recommended.
Description: Rigs with solid front axles, limited slips, sway bar disconnects, and aggressive tires recommended. Driving skill & intuition really starts to matter a great deal at this level.

¨¨¨¨Level 4
Technical Recommendations: Rear locker needed. Front locker / limited slip recommended. Winch recommended. High challenge!
Description: Aggressive tires of at least 31" needed (33 or larger recommended), a rear locker, winch recommended. Your experience should include: steep hill-climbing, traversing ledges and steps, crossing mud holes, winching, log crossings, driving through deep water, large rocks, etc. Good spotters and trail guides can help you get through. Spare parts, tools, and recovery equipment are needed here. Run these trails with club members and experienced folks. Damage and broken parts can definitely happen here.

¨¨¨¨¨Level 5
Technical Recommendations: Front and rear lockers, winch and driving experience needed / recommended.
Description: Nobody should be on these trails without 33"+ tires, locker front and rear, a winch, lots of experience, and a total disregard for the mechanical or cosmetic condition of their vehicle.