1997 LT Blazer

I've been chasing a problem with switching from 2WD to 4WD and reverse when the Blazer is cold. It just won't do it. The lights flash 2-6 tmies and
then the shift never happens. Same if I try to get it into 4L.
If the vehicle warms up (ie: driven for 30-40 minutes) it will shift just like it should.
I suspect that I'll have to go to the dealer for this to solve, but I'm looking for any additional ideas first.
Thanks to all..
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