Transmission problems?

My wife's car had some transmission problems a year ago and we took it into a local mechanic. They said that a seal was compromised, so they replaced it and all seemed well. Ever since, my wife's been complaining that the car's traction isn't what it used to be when she's driving in snow/ice. I don't drive the car that much so it's hard for me to tell a difference, but she swears by it. There's still a good amount of wear left on the tires. After reading previous posts, I'm thinking there may be a problem with the rear solenoid if that's the case, does this need to be handled by Subaru, or can any general mechanic handle it?