Although I'm not sure about if this guy is a buick expert, I will recommend a guy in Oceanside@ Lee Myles Transmissions.
501 South Coast Hwy. Oceanside, CA (760) 722-1851. Ask for Paul Rubke (the boss)
He worked on my 2000 Audi w/ Quattro. And nobody is normally allowed to touch that car except the dealer. This guy did the work, showed me the work in progress, parts, etc. And explained anything I didn't understand, and did me right on the price. No B.S. just quality work and good customer service.
If he can work on a complex system like Quattro, I'm sure he can work on a Buick Regal...