What fluids have you guys had luck with in the rearend? Should I add the limited slip modifier if the manufacturer says their oil already has it? Is the ford limited slip additive okay to use?

 

some gear oil has the limited slip additive already in it, which is why they say you don't need it with their oil.

 

Normally the dino fluid requires the additive and the synthetics do not. The label will clearly state "Contains Limited Slip Additive". If it states that it contains the additive, there is no need to add more.
Not sure on the Ford; we just put some in a buddies car and it was a generic brand.
I use Royal Purple, but it is pretty pricey.

 

I've heard nothing but good things about Royal purple, but at 20 bucks a pop its to rich for my blood.

 

I hear you on the price. We put Valvoline Durablend 80w-90 in my friend's car, it was ~$7 /qt., plus the additive ~$5, right around $20 with tax. Got it all at Vatozone.

 

It's only about $40 to change the rear diff fluid with RP... that includes a gasket. I figure it's worth it since I don't have to change it very often.