Something is wrong with my transmission, but I'm not sure what's wrong.?

I have a 2001 Isuzu Rodeo V6 2 wheel drive automatic transmission with 125,000 miles. If i accelerate too fast, when it shifts from 1st to 2nd it "slips" and vibrates. But if I accelerate slowly then it doesn't mess up. Does this mean I need a whole new transmission, or is there an easier and less expensive way to fix this problem.