TJ you are not eligible for duty free treatment of your personal effects if you send them to Thailand and you are coming here on a non-immigrant visa (so called "retirement visa")
Only Thai citizens who have been resident overseas for more than 12 months are entitled to some limited duty free concessions on returning to live in Thailand.
It really is luck of the draw as to how your container will be treated when it arrives in Thailand. Some have reported no or little duty payable - others have reported horror stories.
One consistent theme however is that people using a carrier (from their home country) who has a good agent at the Thai end seem to get more favourable treatment. This is put down to the agent having long established relationships with Thai Customs, thereby leading to more favourable treatment (ie less duty and/or tea money payable).
I used these people (who have worldwide coverage) when moving my personal effects from Oz to LOS, and was very happy with the service. Others have also reported good results with them on the ThaiVisa forum.
http://www.sevenseasworldwide.com/