Swampy-
The sensors won't throw codes if they're working but not sending the correct signal. You need to measure the resistance of the sensors as per the service manual to make an accurate assesment of their functionality.
The truck is designed NOT to shift into OD unless fully warmed up, BTW. The issue of the fluctuating gauge is a problem and needs to be addressed. Sounds like its a loose connection, which you'll have to trace down...
Ryan
