Well, if there is play in the gears then the gears don't have pressure on them "yet".
the truck is parked. You start the truck and as you press the brake and take the trans out of park, the gears are now at a point that there is no pressure on them. You simply set into gear. Now if the gears have enough play in them (backlash) well the truck hasn't moved right (your foot is on the brake), but you placed it in gear so the torque from the motor is turning, thru the transmission and runs down through the T-case (if equipped) and finally down into the rear gearing. Well there it finds the gears backlash (gap on the gears) with open "slop" and the torque then engages the gears. When the gears engage the pinion gear moves forward (or reverse) into the ring gear (talking about a Hypoid style gear here) and this is the "bang" you hear. It is merely the slop between the gearing when it is engaged and the gears smash together.
Most likely I bet if you remove the diff cover and use a Dial Indicator your going to come up with about .020 backlash (way out of spec's). This is due to ware offer the life of the gears and most likely not replacing the lubricant every 30k miles (15 if heavily towing).
The Durango axle whether 8.25 or 9.25 both have a threaded cap internal right next to the carrier bearings that sets the backlash on the pinion/ring gears. It slides the rear carrier left or right (by loosening one side and tightening the other) and thus setting the backlash (slop between gears).
Last edited by hydrashocker; 12-18-2010 at 11:54 PM.