Bearing or U-joint?

[maroon93limited]

So driving around the other day, I turned on my 4x4 and immediatley heard a rythmic klunking coming from the drivers side front wheel area. It was perfectly in time with the rotation of the wheel, and luckily I was going very slow at the time. I'm leaning towards u-joint, but being that I'll be doing the front rotors soon anyway I was replacing the bearings anyway. What is the best way to check my u-joints, or should I replace them while I'm right there anyway?

 

[Anime]

Get under the truck and look at them. You should be able to see the drivers side joint from behind the axle. You might want to shine a flashlight on it. You might not see anything visual though, other than a badly rusted u-joint. Sometimes the needle bearings inside them just rust away and you can't tell they're bad until you take them apart and find the bearings turned to dust.

if it only clunks in 4WD, and not in 2WD, then it's likely the u-joints. It's possible it could be something else though, you may want to check the gear oil in the front diff. You could get a rhythmic clunking if the front gears are engaged but there's not enough gear oil. The bolts do get loose and the fluid can seep out. It could even be dry. In that case tighten the bolts on the front of the diff first, then add gear oil.