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How is your exhaust mounted? Trans and engine mounts are good? one or two piece driveshaft?
Does it do it under decel? What happens if you put it in neutral? Have you checked the rear end?
Remove the cover and inspect for metal fragments, broken or damaged gears, etc. Check bearings, side clearance and backlash. Then do a gear tooth contact pattern analysis
You need to determine if it's speed related or RPM related. Does it do it in second gear the same as third gear?
huh?