I'm not sure about the diff issue, but I just had my vacuum pump replaced for free by the dealer a couple weeks ago. One day I got little to no brake assistance in a parking lot all of a sudden. We were out of town but I was able to drive it to the closest les schwab. Drove fine on the way there even. They tested it and the pump was going bad, but gm tech support told them over the phone that they are covered under an extended coverage safety recall, up to 150k miles. So I was able to drive it safely home and the dealer here fixed it no charge. It's safe to drive for the most part if the motor is spun up, but if the motor is at low rpm the pump starts to struggle to keep the pressure at a steady level for the brakes, causing you to have to push on the pedal way harder than expected. Obviously a serious safety issue, so i would get fixed now before you drive it much more. If it happened at the wrong moment it could drastically increase your stopping distance at a very bad time. Hit up your dealer and they should take care of it for you!
