You should only have to replace the bearing but if you have the life time warranty,that should be link removed Ridgid before you do any repairs and confirm this.Hope that helps.Good luck.
There are several things that can cause the belt to jump off of the pulleys. The tension on the belt is set by the motor's weight. Make sure that the motor can pivot freely on its mount. If the mount cannot pivot then the motor cannot tension the belt properly.
Put a long straight edge between the two pulleys and make sure that they are in alignment with each other. If not you should be able to make adjustments by loosening the bolts that hold the motor to its mount and then sliding the motor into alignment.
Remove the drive belt. Check the pulleys for any damage. Rotate the pulleys by hand and make sure that the bearings spin freely. If either of the pulleys are damaged or if a bearing seem seized then the bad part will need to be replaced.