After bashing today at busse, my revo's rear diff was acting wierd and making a grinding noise and not getting power to the back wheels. After taking the back end apart to get to it, i took out the old oil and inspected the diff, and i saw no broken teeth or anything. Now i don't know what to do, and i need some new diff fluid. What kind of diff fluid should i get and what should i do to try and fix the problem with the grinding i cannot find.