Does pulling the resonator off the exhaust and adding a glasspack or pipe in it's place offer more horsepower and torque??? I am thinking of doing this to my 98 Dodge Dually 4x4 with an auto tranny.

It won't really add any power, just sound cool. Exhaust mods are usually done to try and lower EGTs so that power can be added.

Some Cummins owners like to lose the muffler and leave the resonator for a better sound. And it helps with EGT as said above. May help mpg slightly.

Mine is straight piped, no muffler. It's a '93 and they didn't have resonators on the first gens. Sounds like a semi now.