Most problems such as you describe - are caused by - uneven load in trailer - improper set up on hitch - I would take the back extension off and leave it in garage - when any hilly road is crossed any thing back of the wheels will take a beating - the extra length and weigh will aggravate the up and down motion - and cause the trailer to weave out of control - i have pulled these things for many many years - with your trailer level on hard ground are concrete, check height to where hitch ball will set - level truck and check height where ball will set - what ever tire's you put on the trailer should be the width of the trail wheels if not your tires will balloon out and not track right- have you checked underneath to see if the frame has twisted I have seen units right off the lot have twisted axles - Just some suggestions from what I have found in 50yrs of pulling.
