at 1st the truck would vibrate all the time and I found it had a bent driveshaft so I had a new one made. It is aluminum, balanced and maybe a little shorter on the length like a half inch with new u-joints also. That took care of almost all the problem but it still had a small vibration at say 50-55mph. I know it is coming from the T-case because it is a manual case and I can hear the light clatter in it and feel it in the handle. He drove the truck a while and a few months later told me it was back worse. I found he really bent the cross member and also the case now had a leaking hole in it from the oil pump. I took the case apart and had it welded where the hole was and inspected all the parts. everything looked pretty good. oh I also replaced the housing between the transmission and transfer  case because that cracked when he bent the cross member. so I replaced the cross memeber and sent him on his way, the vibration was back to being really light again in the case and was noticable at 70mph. now it is back again 6 months later and the bushing in the tail housing has slid out onto the slip yoke and the seal also. maybe it is time for a transfer case.