it is definitely not a ground problem. If it were, your fuel gauge would be pegged to the 3 o clock position. The gauge reads 0-90 ohms. 0 = empty and 90(ish) reads full.
You could start by taking off the wire at the top of the tank, which is the ground. See if your gauge pegs to the 3 o clock position. Do it for both tanks and use your selector switch to see if it is reading the same with the grounds off. If they both read way past full then your ground and gauge are good. If it doesn't move then the problem lies in the gauge.
If gauge is good, then you need to test the selector valve (located on passenger side of frame rail). I imagine this is the culprit. My gauge wouldn't move because the selector valve was stuck halfway between L & R tank in the electrical valve. The selector valve is a two piece valve. One is mechanical that actually switches to direct fuel from whatever tank you have selected. The 2nd piece is electrical. This switches to which fuel sender it is reading from. Unfortunately, there isn't a good way (that I know of) to test if the valve is working correctly. I would just replace the selector valve, they aren't that expensive.
Last, if all that checks out good, then you are looking at the actual sender inside the tank. Dropping the tank is really easy. Take out the 8 bolts that hold the tank mounts to the frame (don't take out the bolt for the strap to the frame mount, they are a bitch to get back in). Use a hammer and flat blade screw driver to take off the lock ring and then replace the sending units, they aren't too expensive.
Here is a wiring diagram for you to look at...
