I saw those links and this is what didn't make it seem like Jeeps are included even though they are specifically listed:
Off-Road Vehicle (ORV) Defined
Also known as Off Highway Vehicle (OHV), ORV’s are any motor vehicle designed or modified for use over unimproved terrain if used for recreation or pleasure off a public way and all legally registered motor vehicles when used off a way. Vehicles over 1000 lbs. such as pickup trucks or SUVs are prohibited from all trails at all times.
Registration
All ORVs must be currently registered in Massachusetts under MGL c. 90B. New Massachusetts Recreation Vehicle Laws require the display of registration on the left and right side of the vehicle. Additionally, motor vehicles registered under chapter 90 must now also register as a recreation vehicle under chapter 90B (on/off road motorcycles, Jeeps, etc.) when used for recreation or pleasure once they leave a public way as defined in chapter 90. Out of state recreation vehicle registrations are not valid in Massachusetts. Contact the Division of Environmental Law Enforcement for registration information: (617)626-1610 or www.mass.gov/dfwele/dle/elereg.htm
It seems that you can register as an OHV, but you can't run it on any trail if the Jeep weighs over 1,000 lbs... Sorry for the off-topic