89 f150

I have a 89 F150,300 I6, dual tanks. I thought that maybe just my sending unit was bad in my rear tank because I could switch over to my rear tank (with a half a tank in it) and run my truck with no problem with the gauge on E for a couple of days. Well then when I switched back over to the front tank (which had three quarters of a tank in it before I started using the rear) and it was almost on E. I think that I was running on that tank all the time even though I had it switched to the rear tank. Could it be cross filling or could the selector switch be bad? Would it be the switch over resivour under the truck?? I would really appreciate some help on this...thanks