geof,
When you use a body lift or a suspension lift, there's more to consider than just getting it into the air.
Steering geometry, drive shaft angles, center of gravity and a few other items get shanged. If you don't make the appropriate accomidations, such as a modified pitman arm, shimming the axles and adding anti-sway bars, you're asking for trouble.
SandCarFan,
Yes. It is possible to dump the air-bags while in motion. You can also adjust for ride height and softness.
However, driving with the air-bags deflated is hazardous to the health of your drive-shaft, rear-end, u-joints, carrier bearings and transmission. This is because it changes the shaft-to-yoke angle, causing a severe vibration in your drive-line.
For this same reason, it is ill-advised to adjust the air-bags for height and comfort unless you're prepared and capable to change the position of your rear diff to compensate.