Bit of backstory:

Two days ago I parked my Jeep in an area with no grass. I went out there the next morning to find it in a large mud puddle and had to use the 4WD to get out onto the pavement. When I got on the pavement I put it into 2WD and drove for about 5 minutes to find another spot to park. All was well.

Today I go out and crank the wheel to turn out of the spot, only to be greeted by the Jeep lurching forward with a clunk sound. I got out and didn't see anything preventing the wheels to from rotating. I think it might be stuck in 4 Hi, but the lever still moves from 2WD to 4WD. I can move back and forth in a straight line without the clunking sound happening. Is it a CV joint that's bad?

 

Sounds like one of the axle u-joints look for rust and missing caps. Grab the axle shaft and see if there's play in the joint if there is any play the joint is bad and should be replaced. Use Spicer or Moog

 

I jacked the Jeep up and tried wiggling the axle shaft but it did not seem to budge. It does have rust though, so it still might be the case.

I tried to recreate the noise by having my sister turn the steering wheel back and forth while I rotated the wheels, but had no success. Could it be something else or should I replace the U-joints on the axle anyway?

 

Update: Can no longer get the Jeep to make the noise. I won't try to fix it if it's not broken.

 

That's what I would do, mine clunks after it's rained and sitting for around a day, once it's driven a few 100 feet the clunk goes away so I'm just dealing with it. Imo we're driving vehicles that are twenty years old so to expect everything to work like new is silly. Those axle u-joints make a lot of noise and will cause some bad vibrations when they're bad. A bad looks like this;