Why does rear suspension system keep adjusting itself? on 2007 Buick Rainier

Vehicle is not sitting low like in some other cases mentioned. Also notice a clunking sound sometimes when going from one surface level to another, i.e. backing out of garage which has a lip onto the driveway. Sometimes sound comes from left front of vehicle, other times from rear right of vehicle.

Asked by for the 2007 Buick Rainier
poss air leak at bags or lines,very common and costly issue
When you say "costly", what's a ballpark repair cost?
if it needs everthing poss 1500.00