First, check to see if the 2-speed can be shifted manually by moving the linkage the diaphragm is attached to. If it works that way, then track down the source of the loss of vacuum to the diaphragm. You could have a faulty shift valve, or a leak in one or more of the lines from the valve to the rear axle.
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!