I am now trying to get my fuel gauge to work. I just replaced the switch on the dash, and now the gauge just barely moves stays just below empty on both tanks. Now I'm wondering if the sending units are bad in both tanks. I don't think it's the gauge, because I do get movement on it. I'm not really sure how to check it though. Anybody else had this problem.

If your fuel gauge, Oil pressure and heat all read low or not at all, it may be the gauge voltage regulator.

you can usually ground your sending unit wire and it will make the gauge read full. I like to use a gauge tester because you can control the resistance to verify it has the proper resistance reading per required specs for the particular gauge. but grounding that wire will verify if it is the sender or wire or gauge.

How do you ground your sending unit?

Grounded out the sending unit and it pegs the gauge to full. I'm assuming that the sending unit is bad?

Either bad or you are out of gas in both tanks

or the ground at the connector is no good.

I just filled up the other day to make sure I eliminated the out of gas, and the ground seems good. The gauge just comes up to under empty when the key is turned on.