As someone who has crossed the border via planes, trains, boats and automobiles, the border is still friendly, but its not a freaking drive up window at Burger King...
If you acquired a trailer from a 3rd party in the US and wish to import it to Canada, you are fine as most of the trailers were also sold to Canadian Forces; so its not on the restricted export list. Just have your paperwork ready for CBSA; I'm not sure if is subject to any vehicle tax off the top of head, but CBSA won't charge you duty on it far as I know..
Now, if your just taking it across the border for a trip, you should have no issues as long as you have a license plate on the trailer if required.