Vent your wheels, swap your tires to Rovers and drive it!
Venting your rims will prevent your tires from acting like balloons and bouncing your creeper around on the rocks, they will be able to deform a bit and grab traction.
Tires are the single biggest performance upgrade you can make on the Creeper.
I'd recomend HB Rovers, they are a great tire.
The difference between the stockers and Rovers was amazing.
Then go read the sticky.
Square away your diffs with JB weld or washers or get the FLM or RMD aftermarket spools I went aftermarket myself, but JB weld is cheaper.
Shrink wrap your pins or get the Lokken or RMD pin locks.
Alot of people upgrade their drive shafts to Traxxas parts but I have not had a problem with my stock parts yet.
After the above I'd suggest building your kit stock (put the motor in when you build the tranny TRUST ME!)and see how you like it.
It's out of the box performance is very impressive.
One of the early champions of the Creeper, 1/4 eliptical toy, comped his in virtually stock form (original shock geometry and mounting points, some aluminum parts for extra strength in a bind) and did very well with it that way.
If after trying it stock, you find there are some aspects of its performance that you want to improve, go back to the sticky to get ideas on how to improve it.
Drive it, get to know it, then improve it if you need to.