Definitely check out the Berg axles that Sean referenced. I've had a few crawlers, but if I were building something totally new for competition, it would definitely have the Berg axles.
The people I know who have switched from an Axial comp rig to a Berg comp rig have all noticed a difference due to the lower CoG and built-in "clod-stall" that assists with steep vertical climbs. I'm not going to switch from my Axial comp rig, but if I did it would be Berg.
If you want something a bit less capable but a lot cooler (scale), then check out the Tamiya LandCruiser crawler
(aka CR-01). That might very well be my next kit just for fun crawls.