There area number of issues that can cause vibrations in the front end. First suspicion is wheel balance. Get them checked and possibly rebalanced. Also loose or worn ball-joints, a worn tie-rod end, pooched control arm bushings and so forth. Also get a good alignment done on it. Toe could be off a bit, and if the control arms were not mounted exactly factory spec, you may never get rid of the front end quirks.
You mentioned that you converted to 4wd from a 2wd? Did the donor diff get rebuilt as in unit bearings, brake rotors, pads etc? Sounds like you may have some worn out front axle components or something loose.
Any time I have worked on my own front suspension I go back after 30-40 miles or so and re-torque everything, and then again about 100 miles after that just to be sure.
Hope that helps.