What 56 pan said.

No codes or SES could certainly point to the switch.

Have you actually tried pulling the shift lever into 3rd when it does its thing ??

Dropping out of lockup will give the feeling that it's out of 4th gear as most of the 6.5 rigs hit lockup at 45 mph and it is a very firm shift and when the lockup is unlocked there is not much difference in feel between 3 and 4...


When it does it's thing try manually shifting to 3rd and see if the RPM go up.

If this is the case, then its the lockup and likely the stop light switch.