Download a service manual and figure it out. I do all of the work on all of our cars... even the UBM (ultimate breakdown machine) that we just got. With all of the stuff that you have figured out, you can do it.
Redline or AMSOil full ester gear oil is about $12 a quart. You cannot get better, but it might need a small bottle of friction modifier which you don't have to use if you don't want. I have a fluid pump if you want to borrow it and do the rear diff yourself... you don't even have to take the cover off... just pump it out and replace the ounces that you took out.