4X4 Engage 94 Blazer Not Working

I have a 94 Blazer 4X4 that will not engage into 4 high. When I stop and put it into NO I can engage it into 4 Low and back to 2 wheel drive. But it wont
go into high range. I hade the module replaced about 8 months ago and it has been working fine until today. Can any one offer some advice on what the problem is. It has me stumped. Thanks Tim
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