Robyn's 76 K10 Project

[rthompson.rth]

Hey guys I gotta a question for you all, I really need some feedback on this one as I am going to have to do all the physical work my self, with a minimal amount of guidance. My question is this:
Recently my 76 4x4 Cheyenne with a Manual transmission started doing some weird stuff while driving. When I get up into third gear the shifting levers start vibrating like crazy. I can hear a heavy metal on metal vibration coming from underneath the truck. Any ideas on what that might be? The front end is new. Checked fluid level in both the transmission and the transfer case, which is the 205 new process, fluid levels are good. Its got new ujoints. Tranny and engine mounts look good. Tranny and transfer case probably have about 500,000 miles on them, but I have never "beat it" off-road. I need feedback please. Clues on how to check on what might be going on. Let me know if you need more info.