I have a glasspack on my 4.0 and it sounds more or less the same as my factory muffler lol
I'd definatly switch it out to a magnafow, flowmaster or something of the likes.
on the plus side, glasspacks are dirt cheap. I picked up my cherrybomb for 29.99.
It won't mess anything up and you can just weld it right in the place of your current muffler