I have a 1998 Ford explorer shift into 4 wheel high on its own

What is your question?

1998 Ford explorer.Randomly the tach will peg out running into the speedometer needle and will go into 4 wheel high at the same time on its own.I can tell it is in 4 wheel hi when I try to turn.I have no warning lights or 4 hi inndicators. Turn the engine off and wait a while and tach is normal and out of 4 high.It will do this after 15-20 minutes of driving. On two occasions the air bag light also came on for a few minutes the went off but still stayed in 4 high.The transfer case has Auto,, 4 High 4 low settings

What makes this problem better or worse?

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How long has your 1998 Ford Explorer had this problem?

4 weeks