Except making a hybrid wouldn't be anywhere near that easy. The packaging alone would make it so much more difficult and impractical than a full EV conversion. Sure, go ahead and attach electric motors to the undriven axle. Wait how are you going to do that? It's going go take a complete redesign of the suspension. Even if you did that your going to need the space occupied by the ICE drivetrain and it's supporting equipment to locate the controllers and batteries. Even if you limit the range to 40-50.miles, it's still going to take a lot of space. By the time you put that much effort into building a hybrid, you're nearly at the work required for a full EV conversion.
Contrary to what someone posted previously, you do not have to start from the frame up, that's how Ford (or who ever built that truck) opted to do it. Generally you would reconfigure battery packs to fit into the existing frame and structure of the recieving vehicle. Actually on a full frame vehicle like a truck, once you remove the original drivetrain and associated equipment, you'll have a whole lot of space for batteries and controllers. The challenge would be figuring out how you were going to do motors. The most practical being an electric motor driving original axle, or swap in a Tesla style motor and axle assembly.