The reply from neale_rs makes a lot of sense to me. When you shift from 4H to 2H, the disconnect doesn't happen instantly. You could put the Jeep on jack stands, turn the wheels and listen for noise. I'd also check the oil in the front diff.
From my YJ manual, this is general and should apply to your TJ:
The axle shaft, differential and pinion gear bearings can all produce noise when worn or damaged. Bearing noise can be either a whining, or a growling
sound. Pinion gear bearings have a constant-pitch noise. This noise changes only with vehicle speed. Pinion bearing noise will be higher because it rotates at a faster rate. Drive the vehicle and load the differential. If bearing noise occurs the pinion rear bearing is the source of the noise. If the bearing noise is heard during a coast, front bearing is the source.
Worn, damaged differential bearings usually produce a low pitch noise. Differential bearing noise is similar to pinion bearing. The pitch of differential
bearing noise is also constant and varies only with vehicle speed.
Axle shaft bearings produce noise and vibration when worn or damaged. The noise generally changes when the bearings are loaded. Road test the vehicle. Turn the vehicle sharply to the left and to the right. This will load the bearings and change the noise level. Where axle bearing damage is slight, the noise is usually not noticeable at speeds above 30 mph.
Based on this description, I'm wondering if it's your rear pinion bearing.
