Correct, pull the diff cover and I clean it out with rags/towels. Then refill through the drivers side plug and check the vent line for blockage while you are doing this. Do not over fill. I have used Redline, Amsoil and Schaeffer’s diff fluid. From my experience do NOT add the friction modifier until you add the fluid of your choice and then go to parking lot and do slow speed figure 8's. If the rear end grabs add some modifier and retest.
I am happy with the #267 from Schaeffer’s with no modifier for front and rear diffs.