Why would Power train light come on with no discernable change in performance?

What is your question?

Driving in 2 wheel drive on a sandy dirt road at 45mph, hit a narrow, deep rut. Wrench light came on. That was 40K miles ago & no noticeable degradation of performance. Ford dealer says solution is to replace the transmission...?! I'd like to trade the car but the value goes down with the power train light on! Any suggestions?