Can you use any gearbox and rear diff on any impreza?
 

I know that you have to match the rear diff to the gearbox, but if my gearbox is on its way out can i replace it with another model number if I get the new rear diff too???

As long you are swapping the gearbox and the diff then yes would be the short answer, but there is one thing more.

Not al diff's accept the rear axles you currently have, so could be you have to swap the crown and pinion wheel off the new diff over to your current diff (so your current axles will fit)