If you've got the stock 45T motor, you could try swapping that out for a Holmes Hobbies Crawlmaster Sport 540 13T - I've done that with a number of builds over the years and it offers a significant amount more rpm, plus is nice and smooth - all for around $15.
You could and/or also try a slightly larger pinion gear on the motor - stock they are 14T, perhaps go up to 17T for example?
I'd also seriously consider swapping to LiPo batteries - just a 2S [shorty] is more than enough, although you'll probably want a compatible ESC (as I'm not sure the stock RC4WD is Lipo compatible?)* - if you go that route the Hobbywing 1080 is an excellent unit - very quiet and programmable with the included controller, for around $40-45.
*edit. seems their Outcry III ESC does have a Lipo cut-off as standard, so you could stick with the stock ESC for the time being too.
Hope that helps!