You'll start to hear a whining noise when you turn, if it is missing. You can just add a bottle if that happens. Just make sure there is enough room, if not you may have to remove a few ounces of fluid from the diff first with a siphoning hose.

 

You will hear a "chatter" when turning especially when turning sharp. You could also hear a slight whine.

If you are thinking they didn't install it then suck out 4oz of fluid and install 4oz of modifier and see if it quiets it down.

 

This forum is such a wealth of information... I love it!

How exactly does one know if the fluid in the diff is good? What color should it be? And how full should the diff be?

I noticed that my diff cover was a little damp (actually just about every truck I've ever seen has a dark spot on the cover from the plug) and I always presumed it was normal and to be expected - especially on an older truck with lots of dirt on the diff cover.

Anyways, I pulled the plug out of the diff and stuck my finger in there, the fluid was just below the lip of the plug hole... And it was kind of a tan color.

 

im not sure how to check the condition of the fluid, but on level ground it should be less than 1/4 inch below the fill hole

 

The color should be an amber color. When it starts to get tinted or when water gets in (fluid will turn milky) you want to change it. Usually the only way water can get in there is if you have been driving in deep water or the breather on the top of the differential housing is damaged. This breather goes into a hose to releive pressure from expanding and cooling of the differential fluid.

You checked it correctly! Like Sharps said, it should be just below the fill plug when you stick your finger in there (about 1/4 inch below). You want to change the fluid about every 30k, but the manual says every 60k. Remember to replace the front diff fluid if yours is a 4X4 at the same time.

Transfer Case fluid is done the same way, you just use ATF+4 fluid instead.