Grab a 3S LiPo.

8000mAh should give quite a bit longer runtime (and speed).

For example:

http://www.amazon.com/dp/B0027GBDCO

Edit: Assuming the truck came with a NiMh battery, you're also going to need a LiPo charger. So basically you're looking at $100+ for battery + charger. You can get a few NiMh batteries instead for that kind of money.