I suppose that could be a culprit. Another thing I would check is the vent hose, make sure it's sealed up at the housing and no cracks or holes in it. Make sure it's routed in such a way that water, dirt, etc can't get in. You may want to replace the hose, they are usually pretty rotten after decades of being in the elements. I usually make them a little bit longer so it can be tied up higher, also loop it back on itself. With the opening pointed down, it should be nearly impossible for any contaminants to get in. As for the pinion being loose, the correct way of addressing the issue would be to pull the differential apart and inspect the gears and bearings for damage and wear. If everything checks out reinstall the pinion with a new preload crush sleeve, seal, and yoke nut. You could try tightening the yoke nut, but you run the risk of overtightening and disturbing the pinion bearing preload. This will cause bearing failure and other damage. It's not a difficult job, it is somewhat time consuming. I would not recommend a job like this to a first timer without some experienced help for guidance.

 

This is sage advice!
I can say for myself I have never done a pinion-anything, but had my nut work loose and walked into a drive line shop to ask. The old timer in there was all too familiar with the 10.25 and said it's a good chance you can just re-torque it for a good long while. They also make a delete kit for the pre-load washer so the nut never works loose, he said they used to do that all the time during installs/rebuilds.
Anyway point is he said 500ft-lbs to tighten the nut. Only way for me to do that was stand on a 2.5ft bar on my trusty craftsman 1/2" drive ratchet, haha. (I figure if it broke I get a free new one). 200lbs at 2.5 ft is 500ft-lbs.

That was some 40k and 2 years ago and it's running well still. I'm not suggesting it, just saying.

 

I went 450ft/lb when i did the seal in our 99, been good so far, about 40k. Another way to do it, is pull the axles and tighten till you get 14-16 in /lbs or resistance. Make sure its just the nut backed off, and not a bad rearend.

 

I have gotten away with it a few times on different applications as well. I was mainly referring to the "text book" way of doing it. Yeah the crush sleeves are a pain if you don't have access to air tools. I rebuilt a 14 bolt GM for a guy, the crush sleeve in that I ended up having to get the crush started in my shop press then finish it with the impact. Like Josh mentioned, the solid spacer kits are the only way to go, much more positive preload and you can hammer them down tight. Like most locknuts, the yoke nut is intended to be a one time use. I have gotten away with a generous amount of red locktite and had no problems.

 

A differential loaded and working will run hot enough to boil any moisture out the vent. Fix the loose pinion problem.

 

i don't think its a loose nut. the ujoint and the yoke has very little up and down play in it. when i let off the gas the rear end shakes a bit but no growling

 

What you are describing, sounds like a bad u-joint or worn out pinion yoke. I would start by jacking up the rear and put the transmission in neutral . Grab ahold of the driveshaft and rotate it back and forth, check to see if there is any play between the cross and caps of the u-joint. Also pull it up and down, and side to side. If it's loose on the pinion yoke, you probably have a bad yoke. If you look at the side of the yoke that the u-joint caps sit into, you will notice a little tab that sticks up; it will be on both sides of the pinion yoke. These tabs keep the u-joint on center, over time from wear and tear these tabs wear out and eventually the u-joint will develop some play. The same can happen with the weld yoke on the driveshaft, the bores that hold the u-joint caps wear out over time and cause excessive play. One quick easy indicator of this the face of the u-joint caps will be shiny, almost polished looking. What happens, as the bores in the weld yoke wear; the u-joint caps will start to spin in the bores. As the caps spin, the back side of the snap ring will shine up the face of the cap. Either of these issues will cause a very noticeable driveline vibration, especially at higher speeds. Just to be on the safe side, I would disconnect the driveshaft and check the pinion for looseness. Check for up, down, and side to side; also in and out. Spin it over by hand, it should spin fairly easy you may notice the brakes dragging a bit. Listen for crunchy sounds, feel for hard spots as you rotate it. This should give you an idea if you have a worn or failed pinion bearing(s). Check the pinion seal for wetness, this can also be a culprit for water contamination. In my line of work, driveline vibration is a known culprit for the failure of transmissions, transfer cases, and differentials. It can get very expensive if ignored. Hope this helps.