well i know im running a /6 but i went thru the same thing...
i had 245/60-14's on the back and wasn't even getting into third with the 2.76's...
so i went to a 3.91 (in an 8.75) and upped to a 235/70-15 (28" tall) and was trapping at 5K and running 65mph @ 3000rpm on the road...
so i upped to a 28x9" hoosier slick (will not fit in stock well even with lip rolled) so i did the SS cuts outs...
but anyways now running a 4.86 gear and trapping at 5600 (last time out it was missing really bad so i was short shifting and part throttle so it will be hire under full pedal) goes 50mph @ 3K now on the road LOL...