There should be a circlip that holds the axle in. Most likley it's inside the old dif. If you bought new axles from the dealer, they won't include the circlips.
On a side note, you managed to twist up the rear axles so your converting to RWD? I'm not sure how transfering MORE power to the rear is going to help you. You probably should invest in Driveshaftshop axles at the very least.