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
I've driven trucks with electric shift and trucks with vacuum shift, there is a learning curve to both. Once you have some miles on either one it becomes second nature, just like learning the little quirks of each transmission. I added a reservoir to the vacuum system when I added the 2 speed pig, the original vacuum system on these trucks has a check valve already installed on the firewall, that valve is the first fitting in line from the manifold. If you install a check valve at the reservoir, it won't allow vacuum into the tank or won't allow vacuum out of the tank depending on the orientation of the valve, the factory check valve holds whatever vacuum is in the system and keeps it from being pulled back into the manifold.
