Since you can't push the truck in neutral and you hear a noise it could be the transmission is okay and the rear differential is the problem. Let raise the rear for the truck (parking brake off on level ground) and see if you can turn the driveshaft.
This guide will help:
If you can't turn the drive shaft, remove the rear cover for inspection, use a flashlight to check the pinion and rings gears. Check out the images (below). Please upload pictures or videos of the problem so we can see what's going on. Have a fluid catch basin ready because gear oil will come out.
Please go over this guide and get back to us.
Images (Click to make bigger)
Monday, December 11th, 2023 AT 12:10 PM