I HAVE A 2001 DIAMANTE, IM GOIN THROUGH MY BACK TIRES VERY QUICKLY THEY ONLY THREAD ON THE INNER HALF..HELP?

I have a 2001 mitsubishi diamante. i've already changed my rear tires a few times in the last 6 months. my tires begin to thread on one side only, and you would not be able to see it unless you bend down and look at the tire underneath the car. my car also feels as if im driving on a sheet of ice when it rains or if the street is wet because it almost feels as if my back end is fishtailing. my steering wheel does not pull to any side when i let go of the wheel it will continue straight. but if any of my back tires hit a fold in the street or a bump my wheel jerks back and forth. my mechanic told me it was my tires but when i put a set of new (used) tires on there it didn't help. he has given me an alignment before but i don't know if it was only for the front wheels. i'm so tired of having problems with my tires...i just want it fixed. but i need to know what to do first. if i get another set of tires and a 4 wheel alignment will it solve my problem? do my tires in the front need to be the same size and brand as my tires in the back? please help!