Driving BMW X5 at 50 mph, front differential locks up taking vehicle from 50 mph to approximately 25 mph in about 1 second then differential fails. Gear oil sprayed on hot exhaust and engine, car would not move forward, passenger (my wife) jumps out when we stopped thinking car was going to catch fire (smoke and strong burning smell from oil). Dealer had car towed to shop, had to replace forward drive train. Car was in shop 4 days. My wife will not ride in car again, she almost impacted the dash when axel locked up, had road been wet or icy, we could have very easily lost control. Had a vehicle been behind us, very likely they would have run into the back of the car it stopped so fast and unexpectedly. Felt like we ran over a large tree in the road but the road was perfectly clear. Immediately traded the car and bought another vehicle, believe ntsb needs to be aware of the failure and see if it happens elsewhere, very scary to have car wheels lock up on the highway at any speed.

- Bellevue, WA, USA