OK, I think I understand what you have bee describing. It sounds like one of two things have happened. The threads on the plug or the convertor have been stripped, or more likely, depending on ther convertor, there is a welded shoulder inside that the plug threads into. If it's just turning and not coming out, then it is likely that the welded shoulder has broken free of the convertor housing. In either case, I am afraid there is no easy way to fix the problem w/o replacing the convertor, or at least removing it to have the work done. If the threads are stripped, I would recommend replacing the convertor, If the welded shoulder is broken free, I would recommend replacing the convertor. Either way, drilling and the like is only going to, at best, get the old plug out. Then what?? FYI, above the plug you will find the stator vanes and drive vanes which do not deal well with drilling or imbalance. My friend, I don't see an easy solution for this one. Maybe someone else might come up with another idea, but I think you are looking at a replacement torque convertor.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.