The front shaft spins whether you are in 2wd or 4wd. It could be a u-joint, or driveshaft angle problem, could also be a bearing in the differential. What lift amount do you have and can you describe the sound a little more.
'95 XJ country edition, 4" coils/3/4" spacers, HD leafs with extra main added in, Ironman adjustable control arms, rough country shackles, aussie locker, JK rubicon shocks, 35" mudder's, soft 8's, cobra cb
Sold '00 TJ, 4" lift, rusty's adjustable control arms, skyjacker shocks, rock krawler adj. trac-bar,rugged ridge SYE and RE driveshaft, soft 8's, 35" mud king xt's,XJ HP D30 w/aussie locker, 4:88 yukon gears, hella 500's, rock crusher diff covers
4. 04 Dodge ram 1500 quadcab, yes, it has a hemi.
5. '08 BMW X3, Mods in progress.
3.5"in lift on 32x11.5s. checked the u joints when i did the left side wheel bearings, they were good. the sound is hard to explain. it is completely unoticeable under 55mph and quickly builds up once at 60mph and as i accelerate higher it gets worse. the jeep is pulling slightly to the left and rides a little rough but i think that has to do with the tire ware and alignment being off slightly. it doesnt create a wobble or shake its just that clicking/knocking sound over 60.
pulled front drive shaft off tonight and although it rode a little smoother, im still feeling the tapping in the front end over 60mph. i tried swerving side to side tonight and it does seem to be on the left side of the vehicle. ive already changed the wheel bearing on that side and checkd the axel while it was out and it looked fine. the only other two things would be ujoint and wheel bearing that i can think of. would either of those be causing a sound like that?