Rachel,
I used to work often with Labrador Retriever rescue. Most often, a group of volunteers would drive the dog to its new location (arranged in shorter segments, where one transporter passes off to the next). I don't have my contacts handy at the moment, but if you are interested, e-mail me directly (pls put "FT" in the e-mail heading), and I will get info to you on how to contact the proper people. My e-mail is in my profile.
God bless you for doing this. I have rescued two labs, and they have turned out to be the most loving, grateful dogs I have ever had.
Robin