1998 XJ, 4.0L, 5 speed stick, ~370,000km. Unsure of the front differential type but I believe it's Dana 30 - I'll know more when I check it out further tonight.

So basically, last week I started to hear a growling/vibration coming from the front passenger side of the vehicle. It starts up at around 30-40km/hr and gets more noticeable the faster you go. Unrelated to engine speed - doesn't happen when you rev up the engine at a standstill. If you leave it in gear and let off the gas going downhill, the sound gets much more noticeable - but if you pull it out of gear and let it coast down a hill, the vibration/sound dies off quite a bit.

First thought was the front hub bearings. Jacked up the Jeep and put it on 4 jackstands,gave the wheel some good tugs and it had slight play, but not a lot. Had a friend put it in gear and slowly give it gas while I listened - a knocking sound was definitely coming from the front wheel somewhere. No sound/vibration if the Jeep was in 2WD, so appears to not be rear driveshaft/rear wheel related. Sound/vibration does not change if 4WD is engaged, at either low or high speeds.

Replaced the passenger hub, the old bearing appeared to be quite good but I had the parts. The vibration was still there, a bit smoother but still there. Pulled it apart again and checked the axle ujoints, there's almost no play there that I can tell... but didn't replace them. Checked the tie rods, no play there either. The brake rotor and pads didn't seem warped.

The diff seals/bearings were redone not too long ago. Front driveshaft was rebuilt at roughly the same time.

I'm almost out of ideas and it's starting to worry me. I'm gonna check the front diff plug tonight for metal shavings; about the only thing I can think of is that the gears are starting to lose it, maybe.

 

You might want to pull the front driveshaft off just to rule it out completely. There are 3 u-joints and a centering socket yoke (in the cardan joint between the 2 u-joints) all of which can cause growls/vibrations if something is going wrong. Even if it was recently rebuilt, it doesn't mean that something hasn't gone wrong (for example, a dry cap in one of the joints...which happened to me on a recently rebuilt rear shaft).