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I'm not sure on this truck. My truck is a 4X4 Silverado and the switch I was referring to enables the PTO function in the ECM. I assume this does the same. I would be curious to see the switch you're referring to, if it's not to much trouble maybe upload a picture.

2) Why would it part time let it shift up to

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If the truck was in PTO mode, as I suspect, it limits the RPM. You probably weren't able to get the conditions right for a 5th gear shift.

James Kuhns
Owner/Technician
Auto Tech Solutions
Chatham, New York, USA