Ford Repair: 08 Ford Fusion, power takeoff, ford fusion

Question
I just bought an AWD, Ford Fusion SEL last June and have had to take it back to the dealership for repairs twice already. When I bought the vehicle the tires on the front didn't match the tires on the back. When I asked if this could cause problems on an AWD car, they assured me it wouldn't. Right now I'm getting the Power Takeoff unit replaced. Before I open up a can of worms with the dealership, I wanted to hear the truth from an outside expertise to see if this is in fact what could be causing all these issues.
Please help!

Answer
first, i've never replaced a ptu on a fusion as long as they have been out !!! why are they replacing it ??? also tires are important... not so much that they are mis=matched front to rear but that they are of the same tread depth !!! as tires wear they obviously become smaller in circumference. if the front or rear tires are more than 1/8th inch difference in tread depth then damage to the awd can occur. the system will think one wheel is spinning faster than the other and try to engage the awd