I knew I had herd that sound before when I wore my rear end out of my 93 I had a 4 cyl turning 35 and over time at highway speeds it gave way and makes a sound like a machine gun and I hate to quote myself but I knew it was the gears in the rear here is my quote from up top
sounds like a machine gun going off
but I would start by looking at the drive shaft area and if that didnt show any signs of damage I would maybe look into the rear most output shaft of my t-case and then go fwd from there it sounds like it could be the rear gear has a few teath broke on it and when the teeth catch back up you can hearit hitting
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