First of all, I seen to have a lot of small problems with Windstar. So I will just bring them out one at a time.

My 3.8L Windstar's Fuel Gauge must've died about 2 years ago. It always reads Full. Check Engine Light is always on since that time. I had the code read at autozone and 2 codes were read. I don't have the exact code right not, but they both read something like bank 1 and bank 2 are reading different fuel levels.

So far the mechanics have not been able to find a simple solution. They wanna change the tank. They estimate the cost to be $500.

Any suggestions?

From the Ford Maintenance Manual:

Fuel Level Sensor

The fuel pump module and pump assembly consists of a variable resistor controlled by the level of an attached float in the fuel tank.
When the fuel level is low, resistance in the fuel pump module and pump is low and movement of the fuel gauge indicator dial is minimal (from EMPTY position).
When the fuel level is high, the resistance in the fuel pump module and pump is high and gauge indicator dial movement is greater (further from the EMPTY position).

If you are having problems with both your gauge and maitaining proper fuel pressure, then your problem likely is the fuel pump module which is integrated with your fuel tank.