One of the simple ways of testing if you will be needing a new transmission is to have someone seat behind the wheel (MOTOR NOT RUNNIG!), and have someone change the shifter selector up and down from P to 3. While this person is moving the selector, look underneath the car just below the driver side and look if the lever connected to the transmission housing is moving. If it doesn't move to approximately 3 position which is the D from park, your transmission lever cable is loose. Take the truck to certified mechanics and have them adjust the lever for you.
Otherwise, you will be needing a new transmission. New transmission is pretty costly, try looking at rebuild options..