That doesn't make sense unless your Titan thinks that when the parking brake is applied you are depressing the brake pedal and allows the shifter to be moved up to the P position. There is a recall about the shifter maybe that is related to your issue. Here is the contents of the recall:
Recall Date: SEP 22, 2004
Potential Number Of Units Affected : 5,600 Read Full
Recall Notice
Defect Summary:
Nissan has determined that some 2004 model year Nissan Titan vehicles may have a defect which relates to motor vehicle safety. There is a possibility that the transmission shift lever assembly for the column shifter may have been damaged during manufacture. The damage could eventually result in the failure of the portion of the assembly that holds the shift lever in the “Park” position when the key is not in the ignition or when the key is in the “Lock”, “Off” or “Accessory” position. This would allow the shift lever to be capable of being moved from “Park” without the key or when the ignition is in a position other than “On”. In some situations, the ability of the shift lever to be moved out of the Park position could result in rollaway of the vehicle and cause a crash.
Remedy:
To prevent this condition from occurring, Nissan is conducting a Voluntary Safety Recall Campaign to replace the shift lever assembly
Click here to view
Good luck...

__________________
~Radiant Silver K/C 4x4 SE~
~Big Tow, Off-Road Pkg, Utility Bed~
~Born 08/04~
~Adopted 09/04~
-I don't have the answers, I'm just a guy who likes to think and share his ideas...