Your question: Can I somehow remove the vacuum actuator from the front axle assembly and use it on the transfer case???
The actuator on the front axle is different enough, mounting-wise and actuator-shaft-wise, to not be easily adapted to use it on the transfer case.
If any of your local salvage yards have any full-size Jeep Grand Wagoneers that were equipped with Selec-Trac, then they have a shift actuator that you could use.
The TeamGW price is obscene, but they are a specialty company, and can charge whatever they think they'll get from someone doing a perfection-restoration on a full size Jeep. The prices for FSJs in good shape are obscene, too, at least from the two FSJ-only dealers here in TX. Its a shame they have a role in the inflated prices of parts like the one you need.
The auction sites might have what you need, try searching for Grand Wagonner transfer case vacuum shifter. There are enough of them out there that someone is bound to be selling that part ,if you can't find it by searching for a listing specific to Eagle.