It works! I believe that part is called the range sensor? Around $20 including shipping from RockAuto.

I checked the resistance of the old one and it was erratic while rotating it.
https://cimg8.ibsrv.net/gimg/www.per...00f73c3c93.jpg
https://cimg4.ibsrv.net/gimg/www.per...8ae03b7c17.jpg
https://cimg1.ibsrv.net/gimg/www.per...0c6e3a1676.jpg
https://cimg9.ibsrv.net/gimg/www.per...18523818fa.jpg
Range sensor is under the larger gear. It has 3 wire connector.
https://cimg9.ibsrv.net/gimg/www.per...3d0d3c42da.jpg
Black wheel is range sensor. AllData has instructions on "timing" it on reinstalling. Just mark the line of the gear against the case mark.

Ok, I'm back working on this truck. I've got a P2068 code, which says "Fuel Level Sensor 2 Circuit High Voltage". If I clear the code the fuel gauge shows correct for about 30 seconds, then goes to zero and that little led low fuel indicator illuminates. This issue wasn't there when I bought the truck, but it's been sitting.

I've never dealt with these dual fuel tank setups before. I found the wiring diagram and it looks like the secondary tank has its own sender (along with another pump) to the ECU.

I'm going to try to measure the voltage at the ECU and see what it is. I found a video that says it's like a 5 volt reference signal. Was really helpful.
https://youtu.be/M31XF3PYmJE

Here is the wiring diagram showing the fuel pump sender:
https://cimg3.ibsrv.net/gimg/www.per...b644193283.jpg
https://cimg4.ibsrv.net/gimg/www.per...3d5902c42f.jpg

Well, I got into the ECU and backprobed that pin and I got a pretty steady 3.70 VDC. Clearing the code didn't do anything.

Now that I think about it, the fuel tank is about 1/4 full. If this is a 0-5V range, maybe my voltage should be closer to 1.25 VDC? 0.25 x 5 VDC = 1.25 VDC?

Maybe I will try adding gas or banging on the tank to see if that floating sensor maybe got hung up inside.
https://cimg7.ibsrv.net/gimg/www.per...76cf47769b.jpg

What'd ya FIND ?
Still have the Dual Tank rig ?

Nothing yet. I did put more fuel in it and drove it a bit doing a few WOT and panic stops. But the fuel guage going wonky still returned. I plan on replacing the fuel sender at some point.

Have another issue with the cluster and parasitic draining the battery, so I want to fix that first.