Check the driveline splines and couplers. Each coupler has a roll pin through it to keep it in place. If the front axle pivot joints become loose from wear, it's hard on the splines. If you have spline problems it will be on the front, as that is where most lateral movement occurs. Try engaging the 4wd,and jack one front wheel off the ground. Try to turn the wheel by hand, slamming it back and forth as hard as you can. If all is well, it will turn a few inches and stop, both directions. You will see the driveshaft turn a partial rotation or so until the backlash is taken out of the gears and splines, then it should stop and not turn unless the rear wheels follow. Do that with each front wheel in turn to help isolate the problem. Should be same result on both sides.