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"I've got over 130,000 miles on my TJ. Do you think I should do something with the u-joints in the front axle?" This was a question I was asked just the other day. My answer was "YES, REPLACE THEM!"
"But they look just fine." My answer...."I don't care, replace them anyway."
How do you know when that u-joint is bad? Is it about to go....will it go the next time out on the trail? Who knows? No one really does, but you can play the percentages a bit and figure out when it may be time to take a look at the u-joints on your vehicle. I do know that doing u-joint maintenance in in your driveway is ALWAYS the preferred location, especially when one thinks about the hassles involved with doing u-joint replacement out on the trail. (some folks are well prepared for this but most folks do not)
U-joints do not last forever. Your use of your vehicle....easy trail driving, muddin', rock crawling, or just a bit of 4WD for those winter roads, will have varying affects your u-joint life. Water crossings can wash away the lubricant and introduce contaminants into the needle bearings. When that happens, the needles grind themselves into dust and your u-joint fails.
Here is a photo of a Spicer 5-760x u-joint that came out of my D30 axle after having been in use for almost 40 months. The grease has lost all of its lubrication quality. The needle bearings are pretty much frozen in what ever it is that is left of the worn out grease....probably a mixture of grease and dirt. There was just a hint of rust forming in a few places. Of the other three caps, one was almost this bad and the remaining two had enough lubricant in it to continue doing its job for a while.
You can see the hint of rust starting to form in the bottom of the cap. This one would not have lasted much longer before the needle bearings started to grind into red dust. I've opened up a cap or two where dirt/dust has come out. Amazingly, the u-joint had not broken nor had the cap rotated in the yoke ears.
On the right is a cap taken from a brand new Spicer 5-760x u-joint. The lubricant is "alive and well" and looks like grease to me. Quite a bit different than the "used" one on the left.
My goal here is to make you aware of an important issue, that being routine maintenance for your u-joints. Your front and rear drive shafts have u-joints as well and require periodic maintenance. I suggest you set up a schedule and routinely examine your u-joints. Having a few spares on hand before you inspect them would be a good idea. If you find a bad one, replace it....and while you are at it, replace the other one too. Then you can reset your inspection clock and both axle shafts will come due again at the same time.