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You should be able to take the diaphragm off of the axle and make a piece to replace it that will lock it in high range. Do not try to rely on the vacuum to hold it in high, It must be mechanically locked to stay in.
Mark
With the diaphragm on the axle and with no vacuum to it, it locks the axle in low. The diaphragm pulls the shift arm on the axle to high when vacuum is applied.
Mark