Should I believe this ?

Put a set of 23's on 2002 Yukon 4WD with 32,000 3 weeks ago, had oil changed 3 days later and was told of leak around skid plate. Took it to dealership today and was told wheel bearing seals are gone and the reason is the 23's and the 305/40 tires being too hard. New to the bigger wheel enviroment but also put a set 23" Rozi's on my other son's Z71 Tahoe. Is this a real problem or should I look for another dealership ?

There could be any number of reasons for that failure but the wheel offset and weight could be a factor. It the wheels are a lot heavier than the stock wheels/tires it will put more stress on the bearings and seals. If the offset of the wheels does not center the wheel mass over the center of the bearings/seals that can cause problems as well.