Here is a link on why ST tires are recommended over LT tires for a trailer. It is from a major tire dealer and contains quotes from a few representatives of tire manufacturers on the differences between ST and LT tires. Please check it out.
http://www.discounttire.com/dtcs/inf...ionAnxiety.dos
In a nutshell, the tire weight ratings for an LT tire do not translate straight over to the tire weight ratings for an ST tire. Others have mentioned getting a bigger or higher rated LT tire is a good start but hard to know for sure.
Regards,