Does anyone know when Ford started limited slip rear end? I have a 68 mustang and would like to upgrade the rear to limited slip. I need to know what year i need to target at the junk yard.

 

they started using limited slip diffs in the early 60,s,,finding one should be fun.

 

You're looking for a "Traction-Lok". I've no idea whether you have a 7.25" or an 8" or a 9" rear but I'll guess maybe an 8", as they are the most common in small block V8 cars.
I still feel like I hit the lottery some 15 years ago when I found one in a '68 Ranchero because they just about don't exist anymore in junkyards.

It's fun poking around salvage yards but seriously, lots of luck with that. But if it's an eight inch, it will work for you, whatever you find it in. learn to spot 8 inch rears by sight. Hard to tell whether it's a Trac-Lok when it's belly down in 40 years worth of dirt and weeds. odds are like 10 to 1 it won't be a Trac-Lok anyway, they weren't really a super common option.
Your best bet are swap meets and Ebay. There's folks on Ebay that sell rebuilt ones, sometimes at swap meets too. Another option is to buy one of the aftermarket limited slip or locking options.
Good luck!