You didn't by chance have a CDL switch installed that was subsequently removed, and then you locked the CD by shifting into low?
I read a thread on MUD
where a person (cool breeze) removed his CDL switch (actually his dash I gather), went wheeling, shifted his 80 into low, and then couldn't get the 80 to unlock the CD when shifting out of low. Has something to do with the continuity of the ckt no longer being there when either the switch isn't installed or the terminating plug isn't installed...Now he never followed up on the thread to say "Yep, not having the CDL switch installed was the cause of the problem" so I'm assuming the person that posted up the fix knew what he was talking about
Might be worth a peak behind the dash bezel if you aren't sure
Sorry, but that is all I got.