My truck is making some kind of clicking noise from the rear when pulling away from a stop. I thought it might be the driveshaft U joints but they feel tight. The rear differential doesn't have drain plugs (or I haven't found them) to check fluid level. The noise goes click, click click and matches road speed. The noise stops at about 15 MPH. Ebrake maybe? Tranny or transfer case? Where to look? Pulling away very easily and no noise. 113K miles