4X4 High Range won't engage

Hi, My '98 Blazer won't engage 4X4 High Range. It worked a few weeks ago, but now when I put it in Neutral and press the Hi button, it blinks but won't engage, it just goes back to 2WD. Low Range engages fine...but I
can't get it into Hi at all. I've tried putting it into Neutral and pressing the button, then putting it into Park to try to adjust it, but this doesn't change anything.
Any clues?
Thx, Dala
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