How about trying one of those girdle-like things that gives you additional back support? That would certainly be the cheapest fix, if it works.
Next, tire diameter is one of the biggest factors in ride quality. Our yard is rough, too. When I mow it with a Kubota L3000DT, I don't notice it at all. When I mow it with a Cub Cadet or Husky Zero Turn, I notice it A LOT. So an older tractor with big tires might work, if you can use it on your yard.
Or, farm out the mowing to someone else. It will eventually come to that for all of us if we live long enough.
Good luck and let us know what you decide.