Did it start to do that all of a sudden or has it always been like that?
Does it do it in 4x4L as well?
Did it first happen when doing some 4X work?

Your symptoms are exactly like how a Mechanical Diff Lock works (Detroit Locker, Lokka etc)

As suggested... If you jack the front diff so both wheels are off the ground, engage the hubs and turn the wheels by hand that will help pinpoint the issue.