Did you double, triple, check the fluid level if it's in compliance with the Spec? Or did you hire a mech to do it for you?
What oil did you use?
Does your transmission have a dip-stick under the hood or is it a sealed transmission? What year is your truck?
I had slippage or hard shift problems from stop to go. It turned out my fluid levels were not right (too low), filling it to right level fixed it. Make sure the transmission is running at the right temperature when checking the fluid on my 2006 the FSM mentions 97F to 115F (you can use a ScanGauge tool to read the sensor readings from OBD, using a heat gun on the tranny case won't be accurate at all). It has to be in a certain range or the fluid can be expanded too much. Also you need to check the transmission fluid while the engine is RUNNING, which pushes the fluid through the system. If you're checking fluid while the engine is off it will look over filled causing you to let too much oil out.
Make sure when checking fluid level, your car is FLAT, if it's on an incline you can risk losing too much oil when checking the level.