The reason you need to PUT THE TRANS IN NEUTRAL, is because of what you have now, locked up transmission or between gear transmission.
When you reinstalled the shifter you pushed the shifting forks out of alignment so 2 gears are engaged at the same time.
You will need to remove shifter again and use a screwdriver to manually put trans in neutral.
Once the shifter is out you will see the two shifting forks, last picture here:
TRS Magazine - Shifter Bushing Replacement
They need to be lined up as shown.
Because the transmissions output shaft is connected to the wheels directly you may need to rock the truck to get gears to release, be careful, when you do hit Neutral truck call roll away