Do the proceeds from a truck sold fifteen days before the zakat due date count as zakatable wealth, knowing that it was designated for transporting the owner's goods and that he possesses other wealth reaching the zakat threshold?

A truck designated for transport is not subject to zakat (definition). Zakat is only due on its earnings (ريْعها) if they reach the [Nisab (definition)] threshold and a full lunar year (Hawl) passes, or on the amount for which it was sold, if that amount reaches the Nisab and a Hawl passes. It is permissible to pay its zakat in advance along with other assets whose Hawl has come due. Selling it before the zakat due date does not affect the ruling.