I have a 1998 f150 4.6 liter. I need to know where the ac orifice tube is located

 

It should be located in the piping between the accumulator and evaporator.

 

Would this behind the battery tray at the evaporator?

 

You can usually see some crimps in the tubing to the evap.. There should be a removable connection there so the orifice can be removed.

Before you get involved in this project, do you have the equipment to evacuate and recharge the system.
If not, you can create more problems than you might presently have..