See Figure 1
The axle shaft universal joints, in closed knuckles, are located in the steering knuckle and are bathed in oil as they turn. To check the fluid level in the steering knuckle, remove the filler plug from the inside of the knuckle. The fluid should be at the level of the hole. If it is not, replenish the supply. Examine the knuckle for leaks if the level is abnormally low. A leak should be readily visible.
This applies to 1966-73 Wagoneers J-Series trucks and Jeepsters through 1970. Later models have open-type steering knuckles.