It is possible that you have a front wheel bearing/hub that is bad. With the steering wheel locked, safely raise the front wheel off the ground. Grab the tire and rock it back and forth while holding the tire at the left and right sides. Do it again while holding the tire at the top and bottom. There should be no percevible movement of the tire. Slowly rotate the tire to feel for any grinding or rumble in the bearing. Even if you do not feel anything unusual the bearing may only be vibrating while under load. The left front typically sees more load than the right front in 4x4 mode. Being 6 years old I would guess that the right front is also worn, just not as bad. Since you have a lift kit I assume you also have oversized tires. The extra weight and loads from the tires can cause premature bearing wear. They may not cause a failure they will just wear faster. On another note did you get an alignment after the lift kit was installed? Altering the suspension can change the alignment. You may want to check with an alignment shop that works on lifted trucks. Some times after the lift the vehicle requires alignment settings that are different from the stock setting.