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GM 6.5 diesel service light

I have a 1994 chevy 1/2 ton p.u.4X4,auto, 235000 miles with an occasional service light on dash turning on . I accelerate and the light goes out. Light returns on coasting. I still get good mileage 19 - 20 mpg. Also has developed nasty oil drip at rear of engine. I cannot find any pluasible location. Could it be the rear main seal? Thank you