rear differential locks up...'88 80Q

[MXHOWES at aol.com]

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Every once in awhile (don't you just love these type of electrical? problems)
my rear differential locks up all by it's self. I know that the button hasn't
been pushed, but
sometimes when I back up, it will automatically lock up. It will only do this
after you have stopped the car and started it up again. The button works fine
when you want to engage/disengage it and it will disengage automatically over
15 mph. Where should I be looking first? switch, control unit or what...if
you have btdt..what was the problem?
Thanks, Jim