When there isn't room enough under the truck to use that 4-foot peice of pipe, I have always used a big pipe wrench on the yoke, braced against the floor and my 3/4 drive breaker bar with a socket on the nut and 12-ton bottle jack under the bar. Use a block of wood so the breaker handle doesn't slip off the jack. Then jack away. Works every time.
On a big truck tractor where you can get a good swing at the nut with a hammer you can swell the nut to break the rust with a big hammer and a big chisel or an air chisel it you have one. The air chisel works wherever you can't get a good swing. Make sure you can get another nut first.
