My 91 4.9L idles high and bogs down when I accelerate. I can't find the MAF sensor and I've replaced plugs wires cap and button also a new fuel filter.

 

Have you checked for vacuum leaks?

 

Did you buy the truck in this shape?
Did the problem just start out of the blue?

There is no MAF sensor on this vintage truck. It is equipped with a Map sensor. It's located on the passenger side firewall, near the cowl.

Have you checked fuel pressure?
The 4.9 needs 45 psi minimum, while running.

Have you checked for codes?
Google has a plethora of information if you search for Ford OBDI for retrieving codes. All is needed is a paperclip. There are YouTube videos out there as well. Just count the flashes.