2000 ford F-150 Transmission Problem

I have a 2000 ford F-150 with about 81,000 miles on it. I took it to the dealer because there was a rumbling noise when driving. The dealer's service department said they checked it and couldn't find anything wrong with the transmission, and recommended that I have the transmission fluid changed. This was at 73,000 miles when the vehicle was still covered by my extended warranty. Well, I had the fluid changed, and just took it back to the dealer as the problem worsened only 8,000 miles later. Now, they tell me that the tork converter is failing, and that the previous problem couldn't have been an indication of this as it would never have lasted another 8,000 miles. Is this accurate?