I just purchased a 2004 Ford Sport Trac 4WD with 87K Miles
I Know the slight noise and vibration I'm hearing/feeling is my Limited Slip Axle (4:10 ratio)...
I've owned other vehicles where the limitied slip / Posi unit was quiet with new additive, I've had others that never got quiet... Never owned a Sport Trac before now though...
My question is: Is it normal for this model? should I not worry about it or should I go ahead and replace the gear lube and LS additive?

 

When in doubt, whip it out!
Since you just bought the vehicle you have no idea what is in there. If you drain the lube and replace it with the correct lube and additive at least you know that part is OK. Let's hope you don't find something in there that shouldn't be in there.
What do you think?

 

I agree, I had pretty much decided to do so anyway. I think I am just going to replace all the fluids to be sure.
Thank you for your response!

 

Changed the lube with Amsoil Severe Gear 75W-140 Synthetic and 4 oz of Motorcraft XL-3 ... all is well now, smooth and quiet....