Have you looked at the shift cable?
Make sure it is secured at the drive end. There is a bracket on the lower drivers side of the drive that anchors your shift cable through a notch in the cable. If that lower end of the cable isn't anchored to the drive, it will cause it to not shift. This is for the outer sleeve of the shift cable. The inner cable needs to be anchored to the shift mechanism on the drive.
I've seen that bracket slowly loosen up and the drive go from shifting erratically, to not shifting at all, once the cable isn't secured at all.
