well to start, when i'm going down the highway, my car has a vibration around 65-70 mph. under that speed the car is okay. my differential is leaking fluid from the rear cover and i have no idea what rear is currently in the car. and i have no idea what kind of gear oil to put in and how much. i'm assuming the vibration may be caused by that and i really need to get it fixed asap. The car is a 1992 firebird formula with a TBI 305 and t5 trans. i just spent a week replacing the clutch and transmission and i really dont want to be under the car again to replace the rear haha. if anyone would happen to know anything on what oil i should use and what rear is in the car, i'd apprieciate it.

 

90w gear oil should do the trick.