since it does try to move did you check the T case fluid level and condition ? these do leak because of a manufacturing defect very common rub inside the T case then it leaks .
fluid needs replacing @40K miles .
then if that is good the codes should be pulled GM drive train capable scanner then this will reveal what is the issue most times.
to replace the encoder as was mentioned lifting the wheels off ground to install is some times required. encoders come in the neutral position so the fork in T case may require to be rotated to align ..
the encoder will not rotate because it is always locked unless the lock has power to it then it releases ..