[Gabe]

To reliably shift a ball-lock mechanism you need to apply quite a bit of force and apply it very quickly (speaking from experience). Too little force and the plunger won't push the balls into their slots. Too slow and ball bearings skip out of their slots. This is why every ball-lock shifter I have seen (including ours) uses pneumatics, and I can't see how a servo would work.

My suggestion is to make the gearbox have a mounting that can work with both servo and pneumatics. This interchangibility will be very good, as well as making your design more appealing to other people.