Have someone crank the steering wheel back and forth while you get underneath and check the tie rods and drag link for slop with the vehicle on the ground (Be safe, make sure it's on level ground, in park, with e-brake on)
Are hubs locked or are they free? If it's only knocking in 4wd w/hubs engaged it's probably a bad axle u-joint. You need to remove everything from the knuckle out to replace axle U joints. It takes a while for a first timer and a few special tools are required, but it's not an overly difficult job with basic mechanical skills.
If it knocks whether or not the hub's engaged or free, jack up the drivers side and give the wheel a spin with the hub free. Listen for unusual noises. Probably needs wheel bearings. If wheel bearings are on their way out, do NOT put the job off. If wheel bearing seizes, you'll have to buy a new spindle (and usually a new hub also) and it makes a simple PM service into an expensive job.
If the wheel bearings have never been checked set aside an entire day and and clean, check, & grease *everything*. A lot of trucks coming off GSA or GL lots have been sitting for years. Shouldn't drive them without at least a basic wheel bearing service.
There have been quite a few wheel bearing failure related posts recently - y'all gotta service these things once in a while!
