* Opposite in function: If you're thinking about the function of a sledge (for moving heavy objects), the opposite might be a wheelbarrow, which also moves things but in a different way.
* Opposite in direction: If you're thinking of the direction a sledge is pulled, the opposite would be to push the sledge.
* Opposite in size: A large sledge might be considered the opposite of a small sled (the kind used for recreational purposes).
To give you the most accurate answer, please tell me in what context you are thinking about the opposite of "sledge."