Drive line noise mystery? Solved

[alborz]

I noticed when the air bags for drive axles are empty completely and the chassis sits on the u-bolt sales, the very rear axle touches the chassis but the front drive axle does not. there is about 1/2" or more distance between the saddle and chassis. and also when the air bags are filled the rotation angel of axle is higher on front axle compared to rear axle. So I'm thinking that when the truck is coasting , there is a flip flap or swing play on rear axle rotation and constantly change of angle which in turn change the angle driveshaft between two rear axles.
do you think this theory is any close to reality?
why not four sales touch the chassis? and how can I fix that?
thanks in advance