While it is in 4 wheel drive or just after taking it out, it makes a real loud banging noise under the front end. I have no idea what it is. Any help will be appreciated. Thank you

Get underneath of it, with a good LED flashlight, and start looking and feeling around. Then, it might be pretty obvious.

I have looked under it several times. The only thing that is remotely loose is the left ball joints. But there is minimal play in them. I still can't find the problem. Thank you

The next step is to take a click torque wrench and start checking everything having to do with the suspension. Wear in upper and lower ball joints is very significant--I would replace them (Moog). Grease them, along with other zerc fittings, with synthetic NLGI #2 grease every 7,500 miles. This is frequently overlooked on trucks.

Ball Joint Replacement. Remove circlip (Hi-Tech 75 snap ring pliers); break loose the ball joint from its mount, by beating the mount with a 32 oz. ball pein hammer 5 or 6 times; remove the lower ball joint from the steering knuckle, using a shop press, air hammer, or ball joint C-frame. Prior to installing the new ball joint into the knuckle, insert the stud into the tapered hole of the LCA and ensure the stud does not rock in the tapered hole. Coat sides of new ball joint with grease, press ball joint in knuckle, and reinstall circlip. I use an air hammer, with a 1-inch hammer tip (Grey Pneumatic CH117), to remove the old ball joints and install the new ball joints with the air hammer or C-frame. I prefer top quality Moog ball joints, which come with a lifetime warranty.

When was the last time you changed the differential, as well as the transfer case, fluids?