Driveline whir and whump problem...

'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.

Have you checked you t case fluid level?

Front and rear drive shaft joints?

Truck lifted or stock?

Thanks stockli. I'm fairly convinced whatevers the bad part is rotating at the same rate the wheels do. Would a problem at either the front or rear drive shaft joints cause a sound/whir/slight rock every wheel/tire rotation? I'm guessing the drive shaft turns a good deal faster than the wheels.