Jaw, the spur gear problem could be the spur holder/ bearings. First thing i,d do is see if the whiole slipper/spur gear assembly has any play in it. Grab both oudrives of the assembly and move it in every direction you can. While doing this, is the spur gear changing alignment (not staying straight)? if so, you could have a problem here causing this vibration. If all looks great at this point, disassemble the slipper and place the spur gear on a flat surface to check for evenness. If the spur isn't straight, that's the problem and is a cheap fix.
For the shocks, I'd buy a new set of the Yokomo shocks (the plastic ones), that way you're sure they are 100% compatible. But id the Losi shocks fit, why not use them? It is for bashing after all...
