The switch you need to look at is the neutral saftey switch on the side of the transmission. It is black and on the drivers side. Its is easy to see just above the lip of the transmission pan.

The switch has marks on it for clocking it.
It may be as simple as adjusting it. Or cleaning the connection on it.
I just replaced mine got the new one at NAPA I think around $20

To adjust the switch just put the park brake on and put the trans in neutral. Then climb under the truck and make sure the alignment marks on the switch are correctly lined up.

Should look like this when properly aligned.