4WD and Bulletproof don't usually go hand in hand
However here are some tips:
In addition to http://4wdrc.com/
, search this forum for the XX4 & XXX4 threads
For the XX4, the best mods are 1)Trinity Aluminum Motor Brace 2)B3 bulkhead Mod(see 4wdrc.com for instructions). 3)Also on 4wdrc check out the King Pin replacements 4)http://amainraceway.com/
take a look at the castor blocks.
Someone used to make carbon fibre shock towers that doubled up with the stock ones but I have not seen these available. I know someone with them and he has broken/cracked one shock tower(but was able to finish the race because the reenforcement kept everything together)
I am sure i am missing something. But this is what I've got off the top of my head.
The keys to success with my XX4 have been to A)not run into anything and B)square up on all big air jumps.
As for the XXX4, setting it on the track may cause it to break.
Hope this helps.