Wellwood also supplied an assist for the winners.
Ondrej Pavelec made 24 stops as Winnipeg exacted revenge for a 3-2 loss to the Leafs last month.
Phil Kessel tallied a pair of goals for the once-hot Maple Leafs, who fell for the third straight time.
James Reimer was pulled in the third period after yielding four goals on 28 shots.
Ben Scrivens relieved and recorded five saves for Toronto.