Rear differential fluid low on vehicle with only 9000 miles

Cocky service advisor! Took the diesel truck to a close friend's shop for an oil change. Certified mechanic, not quick oil change place. They alerted me that there were metal shavings on the magnetic rear differential and that the oil in the rear diff was low. Note this truck only has 9000 miles on it. Right away they tried to say that I was being scammed by someone and that oil couldn't be low. I brought them a sample of the oil with the shavings in it. They first complained that the rear differential was over full. (How do you over fill a rear diff?) Then they complained that we had 10w 30 instead of 15 40 in it. Note, it's summer now! Then they just said that the oil sample was discolored, no evidence of metal. When I asked them to explain why the rear diff fluid level was low in the first place on a new truck they replied that just putting your finger in the hole wasn't good enough. Guess what it took 1 3/4 liters to fill it. I think that was low! I've never been so disrespected by a chevy dealer. They won't get my business again. Oh yeah, my neighbor hunts with the owner, I think I'll contact him next.