The M101's get a bad rap as being too heavy and big for trail rigs. There aren't many trailers out there that weigh less than 1000lbs, even a utility trailer like most of us have sitting around. Now, behind a lower hp FJ40 or a CJ maybe so AND depending on how much you are needing haul one of the M416 style of trailer might make more sense for heavy trail use behind a smaller vehicle. With the FJC I have 240+ HP and it's a bit wider anway so it works. I wouldn't hesitate to pull an M101 behind a 40 on mild trails and sure wouldn't give it a second thought behind a 60 etal.
If you are searching out goat trails in a 40 then smaller might be better but for most of us our 'trail' work is mostly light stuff getting to a camp site or something along those lines.
Just my 2 cents and not worth a penny more.