driverbility problem

have 99 xj,4.0 auto,456 gears,33 in tires,120k miles,long slopes seems to not have the pickup an go,the cat was plug up about 2 years ago so l rodded it,seems to do ok,wonder if that could be throwing the computer out wack,or maybe the muffler could be partly restricted?no ck engine lite,will pass emissions,drove the wfs grand,99 4.0 auto 373 gears an there is a big noticble differce.open for suggestions.thanks to any an all.