Put the jeep up on blocks so all four wheels are off the ground. Start it, put it in 4wd and in gear. let out the clutch or put in drive and cramp the front wheels over sharp. If it jums or you can feel a bump in the steering, it is the U-joints.
If you can't then it is because you are on hard surface (asphalt or dirt) and the wheels can't slip easily, causing them to jump.
It is normal for the front end tu jump when in 4wd and turning if you are on any surface where the tires get good, even, traction.
You can tell if they are bad by further closer, physical investigation. Closely inspect the inside of the caps on the U-joints. There should be nice, non damaged, rubber seals around the caps to protect the insides. Typically these will be damaged and let the dirt in and the needle bearings out. You can turn the wheel a little and rotate the tire. See if the knuckle is turning smoothly. Any binding at all means they are bad.
The vehicle will ALWAYS jump in 4wd on dry surface when turning. It is a physics thing about the circumfrence of circles. However, they should not bind when rotating if off the ground even with the wheels turns sharply.
When you pull them out to replace, be very careful taking them out and putting them back in to not disturb anything, especially the seals. Otherwise you will be tearing apart to replace the seals. Might want to change them at the same time.
Popping in the front end in 4WD is normal during hard turns on pavement and packed/dry trails. The inside tire travels less distance than the outside tire, and since they're locked together in 4WD it causes popping since they're trying to travel the same distance.
A good test to see if theres a problem is to put the whole jeep up on jack stands so all 4 wheels are off the ground, then put it in 4WD, cut the wheel all the way, and hit the gas. You shouldn't hear any popping with the wheels off the ground, and if you don't, you know it's just normal 4WD operation. If you do, then you've already got it on jack stands so you can look at the driveline while someone else (gently) hits the gas. Look/listen for the source of the popping and let us know!
That could be a couple things. First thing I'd check is the diff. Make sure you don't have any worn/broken teeth that are skipping. If you have manual hubs do they turn freely/easily? Any major damage to the axle recently?
Yeah, if it isn't popping on the trails then nothing's wrong. When you put it in 4 you lock the front wheels together so they try to turn the same distance. On any turn the inside wheel travels less, so it needs to slip a little to even it out. That's no problem on trails or low traction situations, but on hard pavement it can't slip as easily and it binds up, which causes the popping and can damage your drivetrain. That's why you always hear people saying never engage it on dry pavement.
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