I own a exeed RC Maxstone 1/10 rockcrawler that has almost nonexistent power when in reverse, Forward is great.

The transmission drives the front and rear wheels, so the only thing that could affect the reverse drive would be a bad reverse electrical feed to the motor (since the control board has to reverse the feed to get the reverse action). Not sure what component they use in the ESC, but if it's a relay, the contacts could be dirty. Otherwise, I guess it could be some tricky gear problem in the transmission, although I can't really visualize how that could happen.
I guess the only other control variable is the radio controller, and maybe the reverse coding is messing up, not engaging the receiver in the crawler enough.
You could run it by a Hobby Shop and see if they've seen this problem before, can test it with a demo conroller.
Good luck, and hope this helps.