I'd say that what you're doing should offer pretty good protection. One thing about tractors vs something like a dune buggy is that you're not likely to wipe it out at high speed and endo it several times. Much more likely is a tip over or maybe a rollover.
The 2x2 tubing you mention is pretty strong - heck, that's almost the size tubing that is used to make receiver hitches for cars/trucks. The key is to try to triangulate _everything_. A triangle is an inherently strong structure. Not so for squares, rectangles, etc.
Best,