My wife is out of town and while she parked her 1997 Mazda B Series pick up truck this morning she stated that her truck continued to roll while parked on a very slight incline. The vehicle was in gear so she put the emergency brake on. Could the clutch be slipping although she said it seemed to be shifting fine. She has to drive back 3 hours over the rolling hills of Nevada and is very concerned that something else could be wrong. What could I have her check besides the clutch fluid?