1996 Chevy Truck Failure to shift

Thanks for the reply. The trans was serviced approximately 9 or 10,000 miles ago. Don't remember the exact mileage. Done at Chevy dealer. Fluid, filter and whatever they do, I guess. Tow a travel trailer, so I wanted to make sure the fluid was changed. Speedo works properly. Check engine light's not on and never came on. I plan to check the OBD's anyway. Problem seemed to start with the colder weather. No temp sensors anywhere other than inside the trans? How about the throttle position sensor? I did set the throttle speed up, as the engine was idling too low after I had the intake manifold gaskets changed at the Chev dealer because of an anti-freeze leak. Thanks, Greydaddy. :