Why would the transmission go bad?
There are just too many reasons for it to happen. Did you start to feeling anything wrong with the car before the transmission went? Like hard shifting and such? Try this site. http://www.aamco.com/troubleshooting/DetailedTransmissionTroubleshootingGuide.html Maybe you can figure out what went wrong by troubleshooting the issue.
Search Chevrolet Tahoe Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Chevrolet Tahoe Experts