On Friday, I bought a used 2005 GMC Sierra 2500 HD with the 6.0L engine. Before buying, I took it to a local garage, and had it inspected. Although the vehicle had high mileage (almost 99K) the mechanic told me it was in great shape, and said he couldn't see anything wrong with it. He said that the transmission fluid looked a little dirty (as if it hadn't been changed in a while) but didn't notice any metal shavings or unusual discolorations or smells. I bought the truck, had the garage change oil, transmission fluid and brake fluid, drove it off the lot, and filled it up with gas. Immediately after filling, the check engine light came on, but I didn't notice anything wrong, so I kept driving. I drove about 60 freeway miles, and noticed a slightly rough shift pattern after pulling off the freeway at my destination. When I arrived, I shut the engine off, visited for a while, and everything seemed fine. Drove about another 50 freeway miles and again noticed rough shift pattern after getting off the freeway. We plugged in our OBD2 reader, and came up with error P0894 - Transmission Component Slipping. We cleared the error code, but noticed it pop back on after about 20 miles of freeway driving. I checked with the garage that originally inspected and serviced the truck and was told they had seen nothing that would indicate any sort of major problem with the transmission. I took the truck to our local Chevrolet dealer (no GMC dealer close by) and they told me that my transmission is in serious trouble and needs to be completely replaced to the tune of $3,322.22. Other than the slightly rough shifting after getting off the freeways (which goes away just as soon as I shut the engine on and restart) I haven't noticed any problems with the transmission. When it's completely cold and running in city traffic, it shifts nice and smooth. The dealer said they were able to diagnose this problem without actually opening up the transmission, by just plugging test equipment into the vehicle. I'm feeling rather suspicious that the dealer may be trying to sell me a new transmission when one is not necessary, so I will take the truck to a transmission shop for a second opinion. Does anyone have any ideas on what might be causing this problem? Do I really need a new transmission, or is the dealer just trying to take me for an expensive ride?