why doesn't my 94 olds 3.8 ltr shift into drive or od?

My 94 olds 88 3.8 ltr. doesn't want to shift into Drive or OD. it will still go down the road at sixty and shift into passingggear. It wants to jerk at speeds of 45 or more but will smooth out at times or at least not so bad, Why? Could this be a speed sensor or another sensor ot tranny problem . IT HAS 165k AND DON'T KNOW ABOUT SERVICE RECORD.