'96 XJ 4.0 4wd here, and what I've been observing for at least the last year is a sort of whirr/whump sound exactly once every wheel rotation, that seems like it could be coming from any direction. At first I thought it was a broken tire band, so I replaced each tire in succession with my spare to see which one it was only to determine it's not any of them. The sound persisted.

Next, since my front axle u-joints were looking bad I thought it might be them. Got some new ones in and the sounds still there.

Last, on a whim, I replaced both my front hubs. Sounds still there.

Now for the first time, what I noticed this week on really smooth road, is that at slow speed you get a slight up and down sensation in the seat in conjunction with the sound.

Now all of this is beyond me, but hopefully there's enough info. here for someone to make a decent guess.

I'm all ears. Thanks.

I would be looking at transfer case mounts and rear drive shaft.

I agree with John. It's likely either: (1) Loose t-case or tranny mounts; (2) Bad u-joint(s) on rear drive shaft; or (3) a bent rear driveshaft. It could also be a bad pinion shaft bearing on the rear diff, but that's not as likely.