My 1992 Rodeo needs a Diff, HELP!!!!!

Hey guys, Im searching for a Diff for my 1992 Rodeo, my mechanic tell me I will need it soon. Question, Do any of you know why it is that my Rodeo has a four
wheel drive light in the dash that is always lit, but no lever shift or switch to turn it off??? I recently purchased the car and I think Im only using two wheels, not sure, how can I tell??? And, finally, Im looking for a web-site that sells parts for my Rodeo.
My respect to all you Isuzu Drivers, Garth

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