New tranny?? Or just speed sensor???

so i bought this 95 monte ls and it was running great there is 150000 miles on it. well on my way home i got on the interstate and at 70 mph i was running at 3200 rpms. so i got closer to home and listened to it shift well it never went into overdrive. so i was hoping that maybe someone could help me out on this one
thanks
roger

This could be caused by an internal problem, or a bad solenoid, it needs a full scan with a trans function scanner to diagnose the problem, my car had a stripped o/d clutch hub, same problem.