Transfer case woes...HELP

[Smallz72project]

So I have a 99 GMC Sierra 1500 extended cab short box its lifted 9 inches. 3 inch body and 6 inch suspension. Shift kit, beefed up motor, and 4.56 gears with a locker running 35-12.50-20s. I keep tearing the transfer case out of it. This is the third time in 2 years that the tcase has gone to crap. It cracks around the adaptor plate. This time its really bad though...cracked completely around and about ready to fall out. Anyone ever experienced this or have some solutions on how to strengthen it. Believe its happening due to natural vibrations that come with driving a vehicle. Second ? is how do I tell what tcase is in it, did they have multiple options or these years only came with one option for tcase. Thanks in advance, I am really getting tired of fixing this all the time. Keep on truckin