I suspect something in the steering box. Might be a crusty or something getting wedged in the ballnut or maybe the ballnut itself is just getting wore out and the balls are getting wedged. Once it starts turning does it steer easily after that (like it just works through a tough spot) or does it just stay hard to turn? Is it hard to steer in both directions?
Usually, if it is something binding in the 4wd knuckle, it will jerk the steering one way or the other. But, if it steers easily with the front end jacked up then it could possibly be something up there causing the problem too.
Have you serviced your steering box recently/ever?
Just be careful cranking too hard on the steering wheel or you can damage the steering box.
Aaron