I had a similar problem, but mine wouldn't go into 3rd at all. I was driving along and it just dropped down to 2nd and wouldn't shift any more. It's an 05 with 105K on it now, 99K at the time (last August). I had towed a camper about 150 miles to the beach and been there a week -- the night before we were coming home the trans acted up! Stored camper and drove it home in 2nd gear, just didn't run over 3000 rpm (right at 50 mph) all the way home. Turned a 2 hour trip into 3, but got home safely. Turned out to be a snap ring inside the trans that let go, a common occurrence according to the shop I took it to. They installed a lot of upgraded parts, including a spiral stainless ring in place of the iron snap ring. Seems like it was about $3500 total for the rebuilt trans, dropped truck off and picked it up three days later. They didn't rebuild mine, but installed one they already had rebuilt... saves time, and it's a common enough trans to keep a couple on hand and rebuild when shop is slow. Gets customers out faster.