Here's a simple way to model an object towed from one end by a rope that can coil up shorter but never stretch longer than a given length.

public class SledFollow : MonoBehaviour {

    [SerializeField]
    [Tooltip("Object that pulls the sled.")]
    Transform _leader;

    [SerializeField]
    [Tooltip("Position on the sled's local z+ axis where its tow rope is attached")]
    float _attachmentDistance = 1;

    [SerializeField]
    [Tooltip("How far from the attachment point can the leader go before pulling?")]
    float _ropeLength = 1;


    Vector3 _attachmentPosition;

    void Start() {
        Vector3 localAttachment = new Vector3(0, 0, _attachmentDistance);
        _attachmentPosition = transform.TransformPoint(localAttachment);        
    }

    void LateUpdate() {

        // When the leader moves further from the attachment point
        // than the length of the rope, move the attachment point toward
        // the leader until the rope is perfectly taut, not over-stretched.
        Vector3 displacement = _attachmentPosition - _leader.position;
        Vector3 rope = Vector3.ClampMagnitude(displacement, _ropeLength);
        _attachmentPosition = _leader.position + rope;

        // If you never scale the sled, you can skip this line.
        float leverLength = _attachmentDistance * transform.localScale.z;

        // When the attachment point moves, move this object as though
        // connected to it by a rigid lever that can't lengthen OR contract.
        displacement = transform.position - _attachmentPosition;
        Vector3 lever = displacement.normalized * leverLength;
        transform.position = _attachmentPosition + lever;

        // Rotate so the attachment point is along our forward vector.
        transform.rotation = Quaternion.LookRotation(-displacement);
    }
}

You may need to make some adjustments if the sled and leader have different heights or go over uneven terrain (such as keeping the sled's up vector aligned with the plane normal, using this function instead of a raw LookRotation), but this gets you the basics.

Here's a simple way to model an object towed from one end by a rope that can coil up shorter but never stretch longer than a given length.

public class SledFollow : MonoBehaviour {

    [SerializeField]
    [Tooltip("Object that pulls the sled.")]
    Transform _leader;

    [SerializeField]
    [Tooltip("Position on the sled's local z+ axis where its tow rope is attached")]
    float _attachmentDistance = 1;

    [SerializeField]
    [Tooltip("How far from the attachment point can the leader go before pulling?")]
    float _ropeLength = 1;


    Vector3 _attachmentPosition;

    void Start() {
        Vector3 localAttachment = new Vector3(0, 0, _attachmentDistance);
        _attachmentPosition = transform.TransformPoint(localAttachment);        
    }

    void LateUpdate() {

        // When the leader moves further from the attachment point
        // than the length of the rope, move the attachment point toward
        // the leader until the rope is perfectly taut, not over-stretched.
        Vector3 displacement = _attachmentPosition - _leader.position;
        Vector3 rope = Vector3.ClampMagnitude(displacement, _ropeLength);
        _attachmentPosition = _leader.position + rope;

        // If you never scale the sled, you can skip this line.
        float leverLength = _attachmentDistance * transform.localScale.z;

        // When the attachment point moves, move this object as though
        // connected to it by a rigid lever that can't lengthen OR contract.
        displacement = transform.position - _attachmentPosition;
        Vector3 lever = displacement.normalized * leverLength;
        transform.position = _attachmentPosition + lever;

        // Rotate so the attachment point is along our forward vector.
        transform.rotation = Quaternion.LookRotation(-displacement);
    }
}

You may need to make some adjustments if the sled and leader have different heights or go over uneven terrain (such as keeping the sled's up vector aligned with the plane normal, using this function instead of a raw LookRotation), but this gets you the basics.