2003 SE, Using factory provided adapter for 4 flat trailor wiring connector. Brake lights, tail lights, side markers, right turn signal all work fine. Left turn signal and hazards freeze after 1 blink when trailor connected. All work fine when trailor is not connected. I have checked all the grounds on the trailor. All appear to be ok. Ideas?

After attempting to debug the issue by validating the same symptoms on 2 other vehicles (1 Nissan, 1 Honda), as last ditch I replaced the bulb in the trailer that correlated to the same side turn signal on the Xterra that was freezing when the trailer was connected. The bulb appeared to be working fine otherwise. Both filaments were working but under the " I tried everything else, I might as well try this" heading, swapping out the bulb has corrected the problem and again I validated the fix across multiple vehicles. I dont know the electrical explanation as to why this would correct the problem, but I am not questioning any further.

Go get a test plug from TSC, or any parts store, It will plug into the flat pigtail on the end of the truck. There are 4 LEDs that will light up and tell you if it is a problem with the truck wiring or the trailer.
http://ecx.images-amazon.com/images/I/31J3235FZ6L._SL500_AA300_.jpg
I would bet that the issue is in the trailer wiring itself, and not the truck wiring.