Not sure about the sentry key Illum. but a search of the innerweb gave me some info regarding the transfer case position sensor. Try finding the connector on that sensor and make sure it is connected and making good clean contact.
91 YJ Sahara, 4.0L, AX-15/NP231, D30/D35@3.07
12 Cherokee 4x4,3.7L,42RLE/MP1522, D30Mfirstname.lastname@example.org
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