I did my axles at 1,100 miles after a long highway trip. They were both slightly underfilled imho, but the oil in the rear came out looking good; the front was another story. It was a really nasty looking mustard yellow color which on the 05 and older is dino. 06 and newer uses synthetic in the front. The rear has been snythetic for a few years. Not really sure if it was worth to drain the rear, but I would defintely at least have them top it off if nothing else. The GM oil for the rear is crazy expensive however, listing for like $27.50 per quart and it takes 4 quarts. I put Royal Purple 75W-90 front and rear and it appears to working well. The G80 Locker works great offroad, and it is smooth with no noise in the turns.