Yesterday leaving work, I stopped at a stoplight and when I gave it a little gas something popped (not real loud) and there was a whirring noise (almost a grinding noise) and the '75 K-5 wouldn't move. Lock in hubs and same thing. Low/high just a loud whirring. Today I crawl under and remove the rear drive-shaft, take the bell shaped cover that the yoke is bolted through. Don't see any metal chips, thrust bearing there is fine, needle bearings are fine. You could turn the (output ?) shaft there and the front drive shaft turned. O.K. put the cover (with yoke) back on with just the bottom bolts to keep everything in place so I could remove the Transfer case and not loose any parts. Started to remove the front drive-shaft u-bolts and the shaft wouldn't turn. ??? So, I locked in the hubs, got in and crank the truck and it was like nothing was wrong. Wanted to pull out of the chocks!!! I think, ALRIGHT!!! Crawl back under and reinstall the other 8 or so bolts to secure the cover and get back in and whirring noise is back and the truck won't move. It is a '75 with a Turbo-350 and 203 transfer case with a part time kit installed by the PO. No problems previously before the "kick out and whirring noise". Any ideas on where to start fixing?