Just upgraded to Goodyear Endurance myself. Increased load range from C to D plus discount tires direct was $430/4 with free shipping. Plus $100 prepaid Visa gift card and sold my Castle Rocks to someone locally for $120. Got the new ones mounted for $30 so overall I think I made out great for the upgrade.
Also upgraded the bearings to Timkens made in the USA the same day and I am glad I did. Bought the trailer new last August and only had 4 trips on it with roughly 600 miles or so. Took off the last wheel and got to the bearing, the cap was never sealed fully from the factory and was full of dirt. Probably would have been disaster on our next trip to Delaware in 2 weeks.