computer display say error in tracking control and 4WD malfunction, the 4wd button beside the ashtray is flashing, i went to see the mechanic, he couldnt see any error on his computer, please help.

When I drive smoothly transmission seems to have no problem with upshifting or downshifting. Only when I press the gas pedal to accelerate the speed, transmission from the 3rd gear stucks, and before it shiftes to the 4th makes this boom noise and shackes the car...

Within the last week, in my 2004 Porsche Cayenne, a warning sign is displayed ("check brake shop" which I assume means that my brakes need to be replaced) so I am going in to a shop to get a second estimate. My question is do I have to get the rotors, sensors and brake pads or can I just get the brake pads replaced? I am very tight on cash right now and don't want to get hosed by a garage. Any tips out there my friends? Thank you, Andrea