In my admittedly limited experience with 2 speed axles, I've found that a vacuum shift unit is a lot smoother shifting than an electric type. Having a good sized reservoir tank with a check valve in it is a necessity, because when a shift is needed on an uphill pull, the intake manifold vacuum is going to be pretty close to zero.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!