My neighbors shop was sinking in the back corner. he had a company that does similar work to the link below do the repair. It took a 1" wide gap in and 2" raise in the concrete floor and made it look like a hairline. The shop had dropped almost 2 inches in the back corner. Once all shop was level again, they drilled a couple of holes in the concrete and filled the space with something to fill the void.
http://www.floridaramjack.com/
One thing to do is to get multiple estimates. his ranged from $7K-22K due to the size of the shop.
Good Luck.