Old JD 953 and 963 gears are excellent for towing and will trail empty at 50 mph straight and true. I think they are only in the 5 and 6 ton range though.

J&M 868 and 1074 are both good gears that seem to trail straight also. I believe the 868 refers to a  8 ton gear with 68"' tread width 1074 is  10 ton with 74" tread width.

Definitely need to think safety if your taking people on rides. Farm Science Review has some nice welded up wagons with rails and enclosures to transport folks out to field demos.

Thought I had some pics but apparently not.