If you've got a place to hook it, a handyman or hi-lift jack would work, but those are really unstable, so cribbing like Mark suggests, along with chocking any wheels still on the ground are essential. With one of those, baby steps in lifting off the cribbing is the safest way to go.
A 4x6 block on top of your floor jack will give you some extra height. But be REAL CAREFUL with that setup.
Or lift at the axle, put cribbing in place on the frame, then let the axle back down. If you're trying to get the axles out or work on the springs, take the wheels off before letting the axle back down to give enough room for the suspension to extend all the way.
