Make sure the alternator hasn't gone bad and is putting out too much
voltage.  I had the alternator flake out in our 94 Trooper...when it did,
you could see the voltage gauge jump up, and the "check transmission" light
came on and it kicked into 2nd gear or whatever for limp mode.  It typically
only lasted a few seconds though.  With transmissions these days being so
electronically controlled, a low or high-voltage problem could cause all
sorts of issues.

If you can get it to drive down the road, I hear Autozone will put a scanner
on it for no charge.  Never tried it myself though...

In this day and age, hooking a car up to a computer should be a normal thing
without a crazy high charge.  Aside from that, any competent mechanic should
be able to think a bit before they run to hook up the computer...

Wesley