i hear noise in my ford explorer rearend do i need to add fluid or what

Your vehicle may have "hubs" on all four wheels instead of a replacable bearing. Check to make sure the hub is not going bad. They are expensive. To check the fluid, remove the plug on the front of the rearend near the drive shaft while the vehicle is level. If fluid leaks out, it's full. If not add until it runs out. When it stops running out, replace plug.