It's not feasable to change trannys, either the regular frontier trans or the hdi. If I were you, I'd go test drive some stuff and see what you think. The new models are more comfortable, stretched wheelbase/less prone to nose heavy, better/thicker seat foam, tires ride softer, quieter trannys.
On the other hand, you could glue your rawhides onto a set of Mudd Ox wheels, run low air pressure, and have a vast imporovement in the bigfoot's ride quality,"suspension", and traction. I replaced my seat foam, made a new thicker, higher seat back, which made a huge difference in comfort.
To Invent, you need a good imagination and a pile of junk. (Thomas Edison)