If it's been used for towing with a high hitch weight, or a lot of cargo, the rear could indeed be sagged. Z71 Tahoe/Avalanche springs will restore the original ride height and 1.5" more. Airlift1000 bags inside your current coil springs can add cheap adjustability. New shocks may DEFINITELY be on the docket if you have 50-100K miles. Or more. On the originals.