Hey!
I'm using the same trailer (the aluminum one) as the base for my project. I used the stock tires to haul some supplies from Sturgeon Bay, WI to Ann Arbor, MI (round trip, about 1k miles). Drove 55 the whole way, no problems, but the trailer was un-loaded.
From what I could find to read about the Norther Tool trailer and bias-ply trailer tires in general is that the main risk is blowout due to overheating (which happens more easily in trailer tires because of the thick sidewalls) when you have the wrong combination of load/hot temperatures/long-hauls. The impression I get is that most/all road-rated (not agricultural) trailer tires are fine up to 55 in normal conditions, and probably fine going faster so long as you're not hauling an overloaded trailer in circles around death valley.
All that being said, just to give myself some peace of mind, I replaced the stock (bias ply) tires with radials. Unless you get a wider axle that will accomodate 13 inch tires, these are pretty much your only choice for the northern tool trailer:
http://recstuff.com/trailer-tires-wheel ... d-range-e/The radials are rated for higher speeds (65, same as all other trailer tires) and are generally just better than bias ply tires (less heat, higher speed ratings)
Ordered from recstuff, I got three tires (left/right & a spare), mounted on galvanized rims & shipped for <$150.
I'm planning on a super-long road trip that's going to involve a lot of long-hauling through hot places next summer which was the main reason I was particularly concerned - I think if I was just planning on cruising around in-state I'd just set the cruise control to 55 and not worry about it. $150 bucks to reduce the risk of a really awful blowout on the highway seemed worth it to me.