twin engines? get two people, one to turn the steering on a trailer the other to watch in the rear, if they both seem to be working good, try to hold one of the still at the sae time as the person trying to steer and see if there is any play, also check the bushing play in the nossels as well, then also check your wear rings, if this is a twin engine when you are up on a plan does it tend to pull to one side?