Changing gear fluids on an uneven surface...
 

Am I able to fill the gearbox and the rear end with just one end of the car jacked up, or do I need to be on an even surface?

It is recomended that the car be level. This is mostly because you may not be able to get the proper amount of gear lube in the car if its not level. I was always told you put it in until it starts to come back out.

I figure that if I fill it with the reccomended amount of fluid, I should be fine.

You can fill it on an uneven surface. Make sure that the car is lifted on the side that will cause the gearbox to be overfilled. Fill until it overflows. Do not replace the filler plug. Place a drain pan under the gearbox and lower the car to the ground so it is level. The extra fluid will drain out. You now have the correct amount of fluid in the gearbox. Raise the car, replace the fill plug, and you are set.

Filling the gearbox w/ the proper amount of fluid isn't the problem.

Making sure that the car is supported properly is. Seriously. Just make sure that the vehicle is supported properly. You don't want the jack stands to slip from their proper positions. (Use some wheel chocks)

i got question when the gearbox is out the car? how do you fill it ??
never did it before