I am having a small vibration when I place my truck in 4x4. I can tell it only happens while in 4x4, because I put it in and out of 4x4 several times and it only happens when I put it in 4x4. It has 130,000 miles and have had little to no trouble with truck so far. I am not especially hard on truck. Brand new tires this week. New brakes and rotors this year. Had front left bearings replaced. This is the first I have heard it, and want to make sure it is not something that is going to get worse or more expensive to fix.

Check drive shafts and universal joints. With 130,000 miles on the truck, one of the u-joints may be bad. If a front u-joint fails, the result can be catastropic if the truck is catapulted into the air end-over-end.

Have you regularly changed the differential and transfer case fluids? If you drain the fluids, when at normal operating temperature, refill them to the proper level for your truck. Use Mobil 1 Synthetic 75W-90 Gear Lubricant for the differentials and Dexron-VI for the transfer case.

If you got bigger tires on the truck they can cause unbalanced wobble.

Or if the front drive line gt bent then you'll need a new one.