Remember that the reverse lockout operates when engine at operating temp, RPM above 608rpm's, and Vehicle Speed above 5 mph (says 5 mph in 2004 Powertrain Diagnostic ManualProcedures Manual...and 3 mph in 2004 Service Manual)
Solenoid on left side of tranny...check to see connections clean, not damaged.
Fused ignition switch output circuit (F202). I believe that this is listed as "Fuse 60...in PDC"...covers both reverse lockout and skip shift solenoid.
Can be open circuit or short to ground in solenoid.
Can also be PCM.
Unless you get lucky on "connections" of fuse (boubt), you'll need DRB III ...and someone smarter than I am for sure !!
Will be glad to fax you info from manual if you think that it will help.