I have a 2002 Mitsubishi Galant, had a mechanic replace the timing belt.

Hello, If the problem you are experiencing happened from the moment you picked up the vehicle from the mechanic following the timing belt replacement, then indeed, it may be related to the work. But, it's so difficult to know what you are experiencing based on your concern description. "Runs like a truck" - meaning, low power? Rough running? Misfiring? Noisy when brake applied -- grinding brakes? Vibration braking? It's too hard to know without being able to get in and take a drive; however, some further description can help too.