In a TH700R4, which is what you have, there is no vacuum control.
The transmission shift timing and shift quality is controlled by a throttle valve cable, that is attached to your throttle body from the transmission.
If it neutrals out from third gear to overdrive, it's most likely an internal problem.
Since the truck is 14 years old and it has the original tranny, I would suspect the 4th gear clutches are worn out.
Given the information that you gave, this would be an educated guess.
The transmission shift timing and shift quality is controlled by a throttle valve cable, that is attached to your throttle body from the transmission.
If it neutrals out from third gear to overdrive, it's most likely an internal problem.
Since the truck is 14 years old and it has the original tranny, I would suspect the 4th gear clutches are worn out.
Given the information that you gave, this would be an educated guess.
