My dog has a Vet phobia..?

My 3 y.o. Labrador is beautifully trained, good natured, and in every way normal and happy. However, when it comes to going to the vet for her immunisations etc, she goes crazy, acts like a different dog, and refuses to go through the door. It is so embarring as I have to literally CARRY (all 37 kgs of her) in, where she then sits cowering and trembling. I have even tried tricking her by going to a different Vet, but she acted the same. I have tried getting her through the door with soothing talk and treats, that didn't work. Growling (not too loud of course) at her didnt work either. (Every-where else we go she has no problems.) Any ideas how to make Vet trips easier on her (and me!!)