Originally Posted by 1966Ford
I changed the fluid in the rearend, and put two bottles of friction modifier in. I drove it about 250 miles this weekend and it does it worse now than it did with the original oil and one bottle of modifier.
Of course it will. You can't just dump in any old amount of friction modifier, there is an exact amount that needs to be in the oil to make it perform right. The friction modifier does just what the name says, it modifies the fricition coefficient of the gear oil. It essentially increases the friction so the clutchs can grab and not just slip. The more you add the harder the clutches will grab making the vibration worse. You're going to need to drain out the oil and refill with the right amount of modifier in it or else you will for sure fry your clutches.
When you drained it did you see any traces of clutch material in the oil? If the clutches are going bad you're going to see it in the oil.