The swepco 210 - 80w140 will do a super job for you in that application. It is a lot more versatile than the straight 90 weight oil is, and will give you a lot more protection. It also has a limited slip additive already in it, so you probably will not have to add the crc ls chemical to it. But to make sure, after you have changed to the new oil, take the unit out to an open area like a parking lot and make slow, tight turns, both left and right. If you sense a "grabbing or hopping" coming from the rear end, then you will need to add the additive to it. Diffs are really odd as you can build them all the same and 2 out of 5 will need the extra help.... Someone told me a long time ago that those are "too tight," but I'm not sure if that is correct or not.
When you are ready to change the gear oil, first take the truck out and bring the oil up to operating temperature - it will be a lot easier to drain when it is warm. Also, it will help get any particles drained out with it. Check your magnet(s) for excessive metal wear and while you are at it, make sure your breather is unobstructed. If you go through water, you should have an extension on your breather so the opening is higher than the water level would be, otherwise you will probably suck in water into the diff and that can cause all kinds of problems if not corrected in a timely manner.
