Strong commutativity preserving generalized derivations on Lie ideals

We apply elementary matrix computations and the theory of differential identities to prove the following: let R be a prime ring with extended centroid C and L a noncommutative Lie ideal of R. Suppose that f:L → R is a map and g is a generalized derivation of R such that [f(x), g(y)]=[x, y] for all x, y ∈ L. Then there exist a nonzero α ∈ C and a map μ: L → C such that g(x)=α x for all x ∈ R and f (x)=α^-1x+ μ(x) for all x ∈ L, except when R ⊆ M₂(F), the 2 × 2 matrix ring over a field F.