lose of power

i have a jeep wrangeler 4.0 L 1999 i am having trubel whith lose of power on hills i am haveing to put it in 3rd on hills that i used to pull in 5th i have ck. all dis whiers and dis. and all the con. it idels ok and rev. ok it soundes like it is muffeld when you put a load to it i need a good place to start......