Back end sits very low - 2002 Oldsmobile Bravada

Big question; did you fit a complete strut (including road springs) on both sides?

Sitting low front or back is a strong indication the road springs are weak, broken or simply wrong. It is a common misunderstanding about the function of shock absorbers - the strut or semi-strut is merely a shock absorber and many have been fruitlessly replaced in the vain hope they will correct a low trim height.

If you replaced the complete assembly both sides it would be wise to measure the trim height. Obtain the official trim height dimension and ignore the false distance after the vehicle has been jacked up - unladen and on level ground and leaving the vehicle in neutral with the parking brake off, roll the car backwards a few feet and then forwards a few feet take the measurements in the prescribed manner.
Measurements could be from wheel arch to the tyre, to the wheel centre or to the ground depending on manufacturer. If the rear is still low after both springs have been replaced then either you have fitted the wrong springs or the front is too high because someone else has fitted the wrong ones.

It is important, incidentally, where a bolt through a rubber bush has been loosened or removed during that sort of work, they are tightened only with the suspension in the road-going condition, ie., with the vehicle on the wheels...

bad shock

Sounds like you have air ride and your system has developed a leak. I suggest getting it fixed soon. You will burn up your compressor and that can get expensive.

Usually when a car sits low its the springs are loose or broken and need replacing.. Broken shock will also lower a cars front end.

your transfer case shifter is locked on 4 low get under take a look at your sifther link shaft, get a long regular screw driver and push the link-shaft up, to release the gear.

Has one or both bad rear air bag's.

Might be a low tire. When they sit over night, and ther isn't enough air in them, they get a flat spot, and it'll stay that way until the tire warms up.

You should check it after the car is warm. Park on level ground
Pull the dipstick out and wipe it with a rag as it will have excess oil on it from the oil splashing. Stick the dipstick back in and pull it out again.

On the dipstick, there should be markings on the dipstick to signify the high and low points (high being the end closer to the handle) the oil should sit in between these two points.

the ride control bags are in the rear only. you probably have a leak in 1 of the air lines or the bags are bad.

You may have either leaking shocks. follow the air line from the shock to the compressor.