Transmission Gear Slipping problems of the 2000 Chevrolet Express

Transmission started to slip when changing gears. The transmission only had 39000 miles on it. I took the van into taylor chev. And was told that they needed me to authorize $1000 in labor costs for diagnostics. I contacted GMC to explain that I felt that they should pickup a all or a portion of the cost. GMC told me that they don't show any records that routine maintenance had been preformed because it was done by an outside source and "that doesn't count". I picked up my van and took it to a transmission shop where I was informed that this is a common problem with the 4l60e transmission. The bearing is not hardened properly and is designed to last for about 40000 miles. The total cost out of my pocket for shabby workmanship on GMC's part was $1678. 00, which is about $1500. 00 less that the dealer told me it was going to cost. I would like to see something done about this.