replace the axle shaft. todays, it is better to replace the axle as it gives you a lifetime warranty on the part.
replacing the boot is cheaper but in most cases, the lack of grease has damaged the joint.
Could be a leak from the differential seal (output seal) This could cause damage to the front gears and bearings. Could also be axle shaft its self, leaking grease from boot. In either case, an inspection would be required