I know this thread is a little old, but I had the same problem. Also ended up putting foam between the refrigerator and the island. Here is my “better” solution.
The root problem is the inside edge of the slide is very solidly supported by rollers when the slide is fully out. This gives the refrigerator a solid footing. As the slide comes in, there are a second set of floor rollers that support the inside edge of the floor and protect the flooring. However, just as the slide comes in fully it tilts slightly, raising the inside edge of the slide floor up off the support rollers. At this point there is no support along the inside edge of the slide. The refrigerator tilts back slightly, but with no solid support under its front feet, the slide floor flexes and the refrigerator rocks easily against the island.
My fix was to make 2 wedges out of wood with rubber mat glued on the bottom to protect the flooring. Once the slide is fully in, I rock the refrigerator back from the top, and scoot the slide wedges tightly under the slide floor directly under the 2 front refrigerator feet. This provides a solid support under the front of the refrigerator and stops the rocking.
The 2 wedges must be removed before attempting to put the slide back out.