I just put aluminum tubes and knuckles on my truck. I got tired od breaking the plastic ones. At $30 a pair for both the knuckles and the tubes, it doesn't take too many broken tubes and knuckles to make it worth it IMO.
You can also use Brett's (CPE) inner and outer axle stiffeners to make the plastic tubes stronger. I just did this to my Son's truck. He took it out today after the upgrade. First time he drove it and didn't break something!
Although from now on, if he breaks a knuckle or tube, it'll be replaced with aluminum. I already did the front knuckles.
If it's the axles themselves that you broke, you can try RC4WD's heavy duty axles. I just put them in my truck as well. I haven't driven the truck yet since the install, but I've heard good things. We'll see...