These are tough online, it's one of the problems where a good road test would go a long way!
The first thing I'd be looking for is a loose tie-rod end.. They can be tricky to pick up when the truck is of the ground, they often feel tight. The best method I've found is to have a helper move the steering wheel back and forth a quarter turn or so, while looking under the front of the truck at all the tie-rod / drag link ends.. If one is loose you'll be able to see the play in it.
The next thing I'd check would be the steering damper.. If it's leaking oil it's suspect. I'd consider removing one end and manually moving it to be sure it's not frozen up.
If everything checks out to this point, I'd jack the front end up and check for any play in the balljoints, and then have the alignment checked.
The worse case scenario is a steering box problem, the toughest part being it's a matter of eliminating everything else.
This is a tough on to really give you a 'answer' on, but I hope at least I've given you some information to help you out. Remember there is no need to click accept unless this information is helpfull to you!
Any questions don't hesitate to reply back and ask!