For Replace 

Launch OpenIV
Enable Edit Mode

[4x4]
Replace Seminole if you want 4x4 Navara.
Replace Seminole at [GTA V/x64e/levels/gta5/vehicles.rpf]

[2x4]
Replace Sadler if you want 2x4 Navara.
Replace Sadler at [GTA V/update/x64/dlcpacks/patchday3ng/dlc.rpf/x64/level/gta5/vehicle.rpf]
and [GTA V/x64e/levels/gta5/vehicles.rpf/]

Make sure to back up your files