One of the biggest issues with rebuilding an auto trans is having a large, clean area to take it apart. There are a lot of parts to keep in order. Also, if there is a good amount of debris inside the trans, the case will need to be thoroughly cleaned. Well-equipped shops will have a steam-cleaning machine to put the parts in and clean. Even the smallest bits of debris can play havoc if they get into the valve body. There are also a number of special tools needed to properly rebuild an auto trans. If it is the snap ring in the reverse clutch that has popped out, you will need to replace the snap ring and the reverse drum, which doesn't come cheap. You will also need a seal and gasket kit and possibly several other seals that don't come in the kit. At this point, it would make sense to check all of the friction plates in all of the clutch drums. I can't speak for one's ability nor how one would fare in a task like rebuilding and auto trans. One thing to keep in mind: if you mess up, you won't know until after you get it all together and then you'll have to take the trans out and all apart, again. That's a real pain in a Hardbody or old Pathy with it's torsion bar suspension. For that alone, it might be worth having a reputable trans shop look at it and, if needed, rebuild it with a warranty.