I've had the same issue with the clunk, here's a little food for thought. many Ford trucks have this same issue (as stated earlier in this post) but I told this story to a friend with a chevy pickup and he was having the same problem he took it into the dealer and they admitted having the same problem with their driveshafts. not sure exactly what they did but $400 dollars later his clunk is gone. I just got my Ford grease in the mail. I'll let you know how it works out