This is a very simple repair, due to the trucks design more than anything.

You'll need to remove the screws holding on the front bumper plate as well as two more underneath holding the bumper on. Pull this plate (and bumper) off along with the aluminum behind it and you'll have direct access to the diffs. Pull the two small spacer blocks out and remove the diff case completely by grabbing the silver CVD's on either side and pull straight out.

Don't be afraid to yank on the diff, there is nothing else holding it in but friction of sand, dirt and a buildup of grime. With the differential removed you can unscrew the broken case (4 screws) and place the inner diff into the new metal case along with the drive-shaft. Apply a bit of white lithium grease to the inside of this new metal case and bolt it together.

Because the metal cases don't mark which side is up you'll need to put it side by side with the original plastic case to figure out which side is up. Slide it into the carrying tracks and insert the CVDs from the wheels to each side of the diff. Remember to align the driveshaft into its yoke underneath the truck when pushing the differential in as well. Gently roll the truck forwards to ensure you installed the diff in the correct way. If it won't roll chances are it's in upside down. Remember to put the black spacers in and finally align and bolt the bumper plate back onto the kingpins.

It sounds more complex than it is really, the trickiest part is making sure to align up the driveshaft underneath the truck as well as the two CVD's to each wheel when reinstalling. I had to replace a plastic case with a metal one last weekend and the entire process took me 5 minutes per diff and I made it in time for the next heat of the race. I guess this is why Losi made the differentials easy to access.