It is a good idea to remove the axle so you can check it is straight and clean the old grease from the bearings. If you have damaged balls see a urolo.... sorry, you can buy new ones at a bycicle shop. If the cones or cups are worn, you are in bigger trouble but sometimes you can go up (best way) or down on the size of the balls to move the bearing surface.
Use automotive chassis grease and do not over tighten the bearings. You should be able to spin the axle between your fingers with no detectable play befor you install the wheel.