I can't go up hills with my car! Is it the transmission?

It always makes this roaring sound if i go up hills. So what does this mean and what exactly am I going to need to get done to my transmission? And approximately how much will it cost. I don't want to get ripped off. Fyi, I have a 1996 Isuzu Rodeo,120,000 miles, excellent condition, except for the trans