Q: I put a new transmission the truck will not move until high RPM

It sounds like a defective torque converter or a bad transmission to me. Double check your fluid level after cycling the shift lever through the gears. Check your electrical connections and the adjustment of the range sensor. Check for codes. If you have an auxiliary fin-style trans cooler, you should have replaced it. Debris from the previous transmission can clog them, and it can not be flushed out. You should have flushed the cooler in the radiator, too. If you disconnect the solenoid connector, you will have 3rd gear at full line pressure, this will help you determine if it is the transmission or the torque converter. If you need more help, consider YourMechanic, as a certified technician can come to your home or office to inspect the transmission.