[justcuz]

Does it have a metal rod that goes from the t-case to the back of the transmission? I agree that a slip yoke eliminator may be your best bet here. You have some kind of leverage or harmonic set up by the rear driveline that is leveraging the t-case and cracking the adaptor. Is you rear drivshaft balanced? Is it dented or bent slightly? All of these things will set up damaging vibrations and take out the weakest link. if you stay with the slip yoke I would lube the splines on the output shaft and inside the yoke with grease and double check your u joints and make sure they are not bad.