Sounds like the high shock has stuck in the extended position and binding. both shocks in the stationary position with no weight to activate leveling should be same height. There should be a static air pressure held in the shocks this is to protect the
seal. about 4 to 6 psi residual. it is held by the pressure check valve behind the compressor. if remove an airline from shock when stationary there should be some air come out. could also be a plugged line that is not venting properly.
Also see if spring has a broken coil at end of spring could have rusted and broke lost tension and sagging.
replace both rear leveler shocks at same time.