Here is the Utah code that regulates registration. This is the section for exemptions from registration.
41-1a-202. Definitions -- Vehicles exempt from registration -- Registration of vehicles after establishing residency
(2) Registration under this chapter is not required for any:
(h) trailer of 750 pounds or less unladen weight and not designed, used, and maintained for hire for the transportation of property or person
these are ofcourse excerpts from the whole code!!!
the only exception to this is a tandem axle trailer, all tandems have to be registered regardless of unladen weight.
Never do anything you don't want to explain to the paramedics.