Hey folks I gotta 85 f150 its got a 9 inch in it and its got a open dif I was wondering can I just buy a new third member that has a dif lock or would I still need to get into it and just replace the open with a locker thanks. Basically I want to kno is the dif lock in the third member

 

If the new diff has Fords LS (limited slip) then its a direct swap, If its an add on locker as on some 4X4 rigs then its a whole new ball game.

 

I have an ARB air locker on my rear diff. Works very well but a bit expensive.
With this you can go from full open to full locked by flipping a switch even in motion.
I had the install done at a shop.
I also installed an air hose fitting on the compressor so I can pump my tires anywhere.

Here is a good blog showing an ARB install on a Jeep front and rear axle. This will let you know what you are up against if you DIY.

http://www.stu-offroad.com/axle/arb/arb-1.htm

.