I have a knocking sound coming from my rear dif only on take off in my 2000 excursion what is going wrong?
The knocking sound could be a broken gear tooth on one of the gears in the differential, a bad universal joint (or velocity joint if equipped), or even a bad bearing.
Removing the rear cover and making a visual inspection should be done, just to see if there are any metal filings in the gear oil.
These hints are just a start. If you need further help, please send us more detailed information on your vehicle.
Let us know how you make out.