Servo......servo.....servo....
I MUST concur that the servo's are the MUST HAVE upgrade and you can remove the dual configuration and replace with just ONE.
Here is what I use and I have NEVER had a failure:
HSC-5955 Titanium Gear High Speed
333oz of torque at 6volts
Dual ball bearings
Titanium-Alloy gear set
Ultra hard servo gear shafts with 2 axial brass case bushings
Water and dust resistant
Metal heatsink, middle case
This MAY sound like over kill, but I assure you that it will be the LAST servo you will ever buy for the truck AND IF you ever have a failure, HITEC will replace it FREE for TWO years!
It will hold the turns on 40 series tires at full throttle and always self centers because it is digital.
Tower sells for $115, but
www.RushRC.com sells for $99.
Anyway, that is my two cents. I have a $2500 Revo that is SICK and I can keep on going with what I think you should do next, so let me know if you want to know.
P.S. I would do the complete aluminum steering upgrade in concert with the servo upgrade. It makes all the difference in the world.