XRAY - Model racing cars

just finished building two xray 808E with the active front diff for a friend and i and 1 xray XT8 truggy with the active front diff converted the truggy to electric using the rc-monster kit we are using the castle mamba monster and the castle 2200kv motor
they were build exactly to match the manufactures instructions the problem we are having is that down the stright the rear is very loose " the rear of the buggy & truggy is like a fish tail back and forth" and we are constantly fighting to keep it stright , we have tried many different tyres same effect.
can you guys give us a few ideas on what to try to help resolve this issue

sure i'll do that

Try without the active diff. It can hurt your driving on bumpy tracks.