Rear End Lube and Additive 96 k1500

Hi all,
I have been told that a additive is absolutely required in limited-slip/posi-traction/locking diffs.
I own the shop manual for this truck and it states nothing about an additive.
The manual sais that a 80/90W oil should be used or a straight 80W in Canada.
Do I require the additive??
Hoping someone can shed some light on this subject.

Forgot to add this a couple of times when I changed diff fluid. Was rewarded with godawful grinding and vibration from rear end. Adding the little bottle of posi additive ($10 or less from any GM dealer) fixes the problem right away!

If you have the typical "Gov-Lock" differential, there is no need for a friction modifier additive. That is only required in cone-clutch type limited slip differentials. since you've had the rear cover off to drain it, the mechanism should be visible. If you see something like this, you don't need any additive:

http://72.19.213.157/files/373carrier3.jpg

Hey thank you for the quick replies.
Blue Bowtie that pic is great!! I haven't started the job yet so as soon as I get the rear diff cover off I will know weather I need the additive or not.
Appreciate the help!