The '02 and newer trucks have a different cluster than the earlier years. It has a digital odometer and a check gauge feature. My guess is the indicator driver has a fail safe built in.

 

That must be it, mines an 02 and will go right back to 0 once I get to 100. My snapon scantool has put the truck moving at 117 before. My digital odometer will read ------ till I back off. I wonder if it still adds up the miles while its like that.