89 F-250 only holds 6 gallons of gas.
 

I have checked the vent hoses from the tank and they are clean. The vent solenoid is seemingly in good condition. Any advice? 6 gallons do not get me very far :(

I try to get as much info as possible before offering any suggestions, sooo , please tell me how you know the tank is empty after using only 6 gallons of gas.

I ran out of gas driving home from work. I haul 2 5 gallon gas tanks with me for that reason.

That's what I thought !
The in tank fuel pump either has a hole on the line from the pump to the tube coming out of the tank or there is a leak internally in the fuel pump module such that when the level goes below a certain point instead of pumping fuel to the engine it winds up pumping air or sucking air. You will need to remove the tank in order to correct this problem.
Just replacing the pump assembly alone isn't the answer unless you can see the leaky part. If you decide to do the job, don't forget to get a new "O" ring for the gauge/pump assy when you reinstall it.
In one way you are lucky because when you go to drop the tank with only 6 gallons out of a full tank , it's heavy. Most of the time when the pump craps out its right after you fill the tank with fuel. Some tanks hold approx. 35 gals.

Heh, yeah, 35 gallons would be a nightmare. I'll be working on this for the next few weeks I guess.

Would it be a bad idea to take the fuel pump out of my rear tank instead of buying a new one? My tank selector switch is broken so I only get to use the front tank.

the front and rear pumps are different on my '91 E 350.I would expect the same with your case,but not sure.

If it were me , I would get the switch problem fixed. Much cheaper and easier.