Low differential fluid in Chevy Tahoe.

[First Sergeant]

Recently, I serviced my new Full-size Chevy Z71 Tahoe and found the axle fluid very, very low. The fluid was then drained and wow! this stuff was absolutely black! Only 7800 miles on this truck. Further, 1.5 qts. came out of the unit. It takes 2.75 qts. I respectfully request the advice of a service professional in order to approach the Service Manager with Knowledge and solid info. First, should I expect damage at this point? Second, the axle did make noise under load and after letting off the throttle. Not really bad but this is a multi-thousand dollar rig! Third, the service manager scheduled me next week and said the fluid color was normal? The axle is a 3.42 ratio limited -slip model and driven by my wife to work. Easy miles... Final, should Chevrolet replace the axle because the fluid was a very gunky black color? Need an experienced rear axle specialist opinion here please. Thanks.