Axle Length

The section/sidewall with is approx. 18.5", but you still need the distance between the rear inner fender lips, taking into account how much clearance you need, and then you need to know the wheel offset.

I don't know how far the axle flanges are past the housing flanges on a 9".

If you really want to get the rear done first, you'll just have to approximate the housing width, order axles for it, then order the correct offset wheels.
Either way, you need to have more measurements and do lots of careful figuring.