4s lipo in a TC3, drive shaft issues

I just bought a mamba 4600 and a 4s 4000 lipo pack for my TC3. I had an old main driveshaft in it that was slightly bent from a previous accident. The first time I revved the motor (maybe 1/4 throttle), the driveshaft bent further (from being out of balance), and started hitting the chassis. I already had a new one waiting to go in, so I have a brand new shaft in there now, but it still does almost the same thing. Free revving it with the wheels in the air, the main shaft starts to bend and and go out of balance by the time I get to about half throttle. It comes back straight when I let off though. Should I be worried about this when I drive it? It might not be an issue at all, since there would be a load on the shaft with the wheels on the ground, but I dont want it to start bending while Im driving, because it could bend far enough to hit my battery (brand new lipo). Ive searched the internet for upgraded stiffer driveshafts with no luck. Thanks for any help/ ideas.