Transmission Probs

My car is actually a 95 Passport. I'd post this on the Passport Forum, but they don't seem to have much to say over there.

My transmission shifts fine through 4th gear. When it shifts to 5th, the dash board "check transmission" light comes on and it won't shift out of it's current gear. Restarting the car resets the warning light. Car drives fine around town, no noticeable slipping. Could it be a problem with the sensor? Anyone else had this problem, or possibly know of a trust worthy mechanic in the San Diego area?

I just repaired an Axiom with the same concern trans would not downshift. But once turned off it would once again start off in first gear, what I found wass an open in what Isuzu describes as shift solenoid A which is in the main pan of the transmissiontwoards the front of the valve body there are two solenoids the one on the pass side of the vehicle is the one you may want to check DVOM should read around .50 ohms or less it is not a bad job but it may be a bear to get the tranny gasket off. If you can get me a diagnostic trpuble code I may be able to better help.