1995 Nissan Sentra O/D light on the dash board is keeping o

Other Category problem
1995 Nissan Sentra 6 cyl Wheel Drive Type unknown Automatic 12900 miles
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Hi,

A orange light with O/D written on it is on continously on the dash board. Problems with my car are:
1. Gear lever is bit cranky
2. Car vibrates a lot during the drive on slightly bad roads.

My other question is what is expected mileage from a car when my specifications.

Any suggestions and answers highly appreciated.