My 2005 Kia sorento ex slips and slides in rain and snow,feels like the all wheel drive doesn't work.How do I fix it?

Tiptoe --- My wife has a 2004 Kia Sorento and has a similar issue on slick roads. We've heard everything from replacing the tires to trying different brakes, pads and rotors. I'd be interested to see what others have to say as it may very well apply to my wife's Sorento, too. One thing I haven't tried, but was just considering --- what about tossing a few 50 - 70 pound sand bags into the back? I used to do that during the Western PA winters with both of my Chevy S-10 trucks and occasionally with my Ford Bronco II. Although, that could help with weight & traction in the winter, it could drag down the fuel economy if left in the vehicle throughout the year....