Red Head Steering Gear has nice rebuilds; http://www.dieselpowerproducts.com/m-183-red-head-steering-gears-inc.aspx They are not the cheapest but it is YOUR life here. Check them out.
:welcome Robert Maraugha! Fill out your signature so we will know what truck you are talking about. Go to User CP to do this. Now, on to your problem. Remove the SuperChips MAX and see how the truck re acts. Any change?
Yes it was the fill level plug. Most people remove the cover to drain the gear oil, I do. Your choice. I do not know anybody who has done that. Maybe someone will chime in.
Is the cover pressed sheet metal? If so that would be very difficult as the factory has a metal bung they press in that is drilled. You could make one and weld it into the cover. :thumbs
If you are having no problems, leave it. If your posi isn't engaging then you can change the oil and add a friction modifier.
FORD XL3 FRICTION MODIFIER ADDITIVE is available at your dealer.