Flywheel in 97' Ranger

I have a 97' 4x4 4.0L automatic ranger. My old motor in my ranger had close to 200,000 miles on it, so my uncle took it out and put in a 97 explorer motor with 47,000 miles on it, in it. If im not mistaken its the same kind of motor and all, I dont know much about vehicles besides checking the oil and keeping gas in it. Since having this motor swap, we've had to replace 4 flyweels due to the fact that after about a week of being fixed, they break just around the center of the flywheel, where it bolts on. Do you know what could possibly be causing this to happen? Ive been told we could be putting the ranger starter on the explorer motor or that it could be we need to replace the bushings. Since im not sure what a bushing is im not sure where to start. We're going to take it to a local mechanic but, I was hoping to have some info. To tell him about my problem, with this being the only vehicle I have, help would be GREATLY appreciated. Even if you dont know exactly a suggestion would be great! :) Could you help me?