First check the fluid level when HOT on flat ground. Check in neutral with engine running and select all gears for at least 2 seconds to get any air bubbles out.
There is a way to see if your issue is electrical or mechanical on your transmission 42-44RE. Testing instructions for 42RE, 44RE, 46RE, 47RE.
By doing this you will not have an automatic transmission, you will have to shift it completely manual and you will have no Overdrive!
Get under truck on the drivers side and unplug the 8 pin harness from the transmission near the linkage input into the trans, (this is the main plug to the transmission). Start your truck and back out or what not. Now drop the gear shift selector without stopping all the way down from reverse (or Park) to 1st and hit the gas. You should be going now running in first gear. Now shift with the selector to 2nd gear, then into Drive. If everything shifts fine then your issue is electronic, if it shifts the same then it is mechanical.
If the problem is electric:
1. Replace the Output Transmission Speed Sensor.
2. Check the 8 pin harness you disconnected for corrosion.
3. Replace the governor solenoid and transducer on the valve body.