If I could do only ONE thing, I'd change the rear end gear. Especially since you have a muffler, and air filter you are satisfied with. It will NOT be cheap though. You did not give a cost limit. What gear ratio do you have?

Of course make sure everything you have is working properly. Wires, plugs, sensors, etc.

 

The rear end is 3.9. Either that or 4.10 is best for towing. But didn't want to kill MPGs when not on the road. So I opted for the 3.9. I think I'll stay there. But, maybe...
...a different torque converter!?

 

That's a good idea.