I received a diesel L4N71b from member MidnightZ last summer. Whilst cleaning it up, I discovered what could be a hard-to-find problem. This is the hard line to the transmission's vacuum modulator:
(click on any image for larger)
While this is not quiet worn enough to leak, it's not far away from it. If you get odd shifting, and the usual suspects aren't found (cracked soft line at the vacuum pump, at the vacuum valve on the IP, the soft line at the modulator on the trans), you might look under this mounting tab on the hard line.