So I'm about to change the fluid in my rear differential. It's a 2000 F250 Super Duty 4x4. How do I know if it's a limited slip differential as if it is I am supposed to add an additive.

Thanks

 

You can check the door tag. The axle will identify if limited slip. Not sure about 2005 but my 2015 shows 3L for the axle code. L identifying a limited slip.

Or you can jack up one rear wheel and try to turn it. If it turns freely, it is not limited slip.