I have just checked my differential oil and found that the rear looks pretty new, but I have some questions.

First, when I loosened the fill plug it sucked or hissed air. I know there is a vent, so I am wondering if the vent might be clogged.

Second, it was a bit foamy on top of the fluid and it had been sitting for at least an hour. Is that normal or bad?

There were signs that is had been very dirty before, the plug had a layer of dark, nasty, greasy stuff under the layer of light brown clear stuff that smelled like new stinky heavy weight gear oil.

I checked the front and it was grey and the fill plug was very dirty like it hadnt been checked in a long time. The rear plug had blue paint on it just as the transfer case had but the front differential had no (inspection) paint on it, just years of crud.

89 4x4 22re automatic extended cab. Thanksfor your replies.

Vent normally will hold a slight vacuum. Why? It is a "jiggle" type valve, kind of like on a pressure cooker. Vibration while driving and the normal build up of pressure as the oil heats up will force air out the vent. Stop moving and the valve will seal off the opening to keep stuff like water and mud out. Normally OK, but if you are stopped in deep water a decent vacuum can build up and possibly suck water in past seals if any are weak:

http://www.4crawler.com/4x4/CheapTricks/WaterProofing.shtml#Breathers

To add to 4Crawler's post, where are you located? You can update your profile via the 'my home' link at the top of the page.

This gives us an idea of your truck's previous history. If it was a mudpuppy, then draining the pumpkins and putting in some breather extensions might help.

REMEMBER--take the filler plug out FIRST, then the drain plug, or you'll be saying bad words! <img src="/forums/images/graemlins/kewl.gif" alt="" />

NOt a mud puppy, just a farm truck

Most of us run vent lines to avoid the oem vent getting stuck in the open/closed position, more of a problem if closed since the air pressure build-up caused by heat can't escape through the vent so it finds the next weakest point, i.e., the axle seals. You can do it for under $20, depending on how much hose line and new fittings cost you.

Well, I checked the rocker style vent and it was clogged with mud, so maybe it was on a muddy farm. Arent they all. I will keep it clean.
So what about the foam? Is that something to worry about?

The foam MAY be water that got sucked in the vent by the vacumm everyone is talking about. Read 4Crawler's site!
Drain the gear oil, do the vent extention, refill the gear oil. Check again in a few months!