Can't engage 4wd in '95 Jimmy

We took our Jimmy to the mechanic because we couldn't get it into 4 wheel drive. The mechanic checked the switch on the dash, it works
fine. He checked all the wiring and vacuum connections; they're all fine. The shop that rebuilt the auto transmission several months ago (4wd was working before then) spent a couple days retracing what they did & THEY couldn't figure out what's wrong.
The mechanic tried getting it into 4wd & was successful at one point, but then couldn't get it back into 2wd. He said all of the dashboard switch lights were on.
Since neither he nor the transmission shop could figure out what's wrong, he suggested taking it to the dealer. He suspects it might be some expensive-to-replace electronic motor or something.
I know enough about engines & standard transmissions to get into trouble, but I know NOTHING about 4 wheel drive. Any input would be greatly appreciated.
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