Jeep Stroker Killing Starters

So I have been having trouble with my stroker eating up starters. The engine has been in for about 2 weeks not and just killed the 3rd starter. The first one was old and just died. We put quite a load on it so that one was kind of expected. The second one died when the Jeep backfired and broke the shaft off the end of it. The third one we put in it was in really good shape (although it was a used starter) and did really great for over a week. (first two only lasted a day each) This morning the Jeep wont start. The battery is relatively new and has had no issues. The starter started to slow down about 2 or 3 days ago and today it wont turn the engine over. Any ideas on why my Jeep is going through so many starters?! :confused: I am starting to run out of spares and don't really want to have to buy one...