Fuel guage went from full to empty, and check engine light came on. Runs great!

It sounds like the fuel level sensor is defective.

Yes, the sending unit is probably defective. Oil pressure varies at different engine RPM, oil viscosity and engine wear. Sounds OK to me. DT

A loose connection on the sending unit could cause the problem but also moisture in the tank in cold weather could also be a factor in which i would add fuel line antifreeze to a full tank of fuel..........Hope this helps you out.

It could be a bad fuel gauge in the dash unit, but....
Usually it's "The bad".
The fuel level sending wipe arm and windings on the fuel sender wear and send erratic readings.The only cure is a replacement fuel gauge sender.
The check engine light should not be related to the erratic fuel readings.
"The good".
If you have over 200,000kms or 120,000 miles on your van you need a new fuel pump anyway,so have a complete assembly put in which will have a new fuel sender on it.

Check the fuses first, possible the sending unit on the fuel tank is bad.

Fuel pump and the sensor that detects the gas level are both inside the gas tank.

sounds like a bad ground in the wiring.

It sounds as if you have a problem in the switcher valve. Make sure front tank is full! There is a problem with the way these are origionally set up. I have seen where the guages showed full but were empty. If the valve is damaged it coould have been using the front tank with the rear also, although it is not supposed to happen it did on this particular truck.You may also want to check and make sure the fuel is clean in that tank.

on your last tune up you might have gotten some bad plugs i know they are new but sometimes they dont work straight out of the factory and your fuel guage connections are backwards

i have had this problem before these trucks are famous for the fuel gauges not to work in the back of the dash panel there is a nine volt battery that runs power to it and might be dead