An axle from a larger truck would be no problem to fit under your frame. However the ratio would be way to fast because todays trucks are geared for 70 mph at about 1500 RPM, Fitting an air ride suspension would be relatively simple too. However, dumping the suspension will only lower you 4 to 5 inches. We have suspension dumps on all of our trailers to ease getting under a loaded trailer. If you will look at your truck now and imagine removing the rear springs, that is as far as you can drop before the axle housing hits the frame. The lift axles you see on trucks are dead drop axles, not drive axles. Also what are you going to do for air? When you drive, the airbags are constantly inflating and deflating to compensate for the road. This takes a lot of air.
