The shift motor linkage includes a spring which is tensioned when a shift is pre-selected, and allows the low/high shift to be made by simply releasing the power momentarily. If the springs are weak or broken the shift will not be made fully and the gears will grind as soon as power is applied. Open up the shift motor assembly and look for weak or broken springs.
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!