Can you not get the Frontier Pro-4x in a longbed? The Nissan Titan used to have that option on its 7' longbed option.
Remember a open/limited slip 4x4 system will send the bulk of the power to the tire with the least amount of resistance. A locker will prevent you from just spinning a single back tire while the other remains motionless.
For me the locker option on the Titan was one of the main reasons I went with a Nissan instead of staying with the Toyota Tundra.
My old pickup would get stranded by the limited slip rear differential on a regular basis. Even with the light application of the parking brake.
The Titan has no such problems and in 4Lo with the Locker engaged can motor out of the same situation without excessive wheel spin, which can lead to parts failure.
-Mack