how do i check the differential oil in the rear axle on 1992 Isuzu Rodeo

my rear is making a popping sound, someone said to check/add oil in the rear differential

Asked by for the 1992 Isuzu Rodeo
There's a plug on the back of the diff for checking and filling and one to drain it. The fluid should about come to the rim of the filler hole once you get the plug out. If the fluid's black or burnt smelling you should change it. Check it cold, not driven, and drain it hot. Use 'GL-5' rated gear lubricant.