I'm having problems with my truck, its not changing speeds normally and it doesn't switch into 3rd speed, also it doesn't go uphill anymore it just accelerates, what could be the problem?

Have the TPS (throttle position sensor) checked, the ECT (engine coolant temp sensor) checked, the TV (throttle valve cable) and always check the basics like trans fluid condition and level checked.
That's a start.

Additionally, check the Vehicle Speed Sensor. When not replaced, vehicle may experience loss of power and/or inaccurate transmission shift points.

Yes, the VSS in the speedo head gave us headaches.