Car sits very low in rear each morning until I drive it.

Your rear load level air bags are leaking. I'm not saying your air bags are bad. The proper diagnostics has to be performed. Take to a pro or open up your wallet.

this is very common after a few yrs have it ck'd for leaks could be bags or air lines. bags with air lines could cost approx 1000.00

you have airbags for a rear suspension and the chances are that you have a small leak in one of the bags so overnight it leaks out lowers the rear end and then when you start the vehicle the air compressor pump it up again so full again I had that problem replace the bags and solved the problem