I had this problem in my 82' LB and it was the U-joint. It was in 4th but worse in 5th because of higher rotation speed. It was associated with a lot of vibration. Did you bang your driveshaft on anything, forget to lube, etc.. Is your noise also associated with vibration like mine was? Open up the drain plug on the diff and dip it in solvent, take the metal "fuzz" and rub it between your fingers. If there is any "sand" in it you have a gearing issue. Do the same with the trans. Good luck!
