This past year I used Interstate Auto Movers to transport my 1993 MSHP SSP to Minneapolis and back to Detroit. This was when I had it sent off to company used to strip and repaint it. They were very reliable and the prices were competitive.
http://www.interstateautomovers.com/
In the past I've used DAS Auto Transport. Personally I didn't have any issues with them, although, I have heard of others who have had issues.
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Mike
1992 Michigan State Police SSP Mustang #5114
1993 Wisconsin State Patrol SSP Mustang
1993 Missouri State Highway Patrol SSP Mustang P641
http://members.fotki.com/MOstang/