Start by removing the round inspection lid under the false floor in the hatch. It is to the right of the tire well near where the tools sit. Once you remove the cover, turn the lock ring, unplug the connector and remove the fuel sender. With the sender out, shine a flashlight down in the tank (not a trouble light!) and find out what the bottom looks like. If it's gunky or rusty, it's time to drop the tank and clean it out. Soak the fuel tank straps for a day or two in PB Blaster or other penetrant and remove the tank. The hoses can be tricky, but they will come off.
Once the tank is down, order the POR-15 fuel tank kit online from the POR15 store and follow the directions EXACTLY! It takes several days to do right, but will fix your tank for good (only if done BY THE BOOK). Here was my tank before and after. It is a messy, no-fun job, but the results will be worth it.