2005 Nissan Maxima P0780 Fault: Shift Malfunction and Motor Mount Wear

I've been experiencing a shift malfunction between 2nd and 3rd gear in my 2005 Nissan Maxima for several months. The issue started gradually, with the transmission shifting roughly and requiring significant force. After inspecting the vehicle, I noticed that the motor mounts are severely worn—there's noticeable movement of the engine when driving, which suggests a potential contributor to the shifting issues. However, I believe this alone does not fully explain the P0780 fault code. I plan to replace the front motor mount tomorrow (August 15, 2009) and will update with results. Does anyone have experience troubleshooting or repairing transmission problems in Nissan Maxima models, especially related to P0780? Any insights on common causes or fixes would be greatly appreciated.