  Noncommutative differential calculus on quantum Minkowski space is not
separated with respect to the standard generators, in the sense that partial
derivatives of functions of a single generator can depend on all other
generators. It is shown that this problem can be overcome by a separation of
variables. We study the action of the universal L-matrix, appearing in the
coproduct of partial derivatives, on generators. Powers of he resulting quantum
Minkowski algebra valued matrices are calculated. This leads to a nonlinear
coordinate transformation which essentially separates the calculus. A compact
formula for general derivatives is obtained in form of a chain rule with
partial Jackson derivatives. It is applied to the massive quantum Klein-Gordon
equation by reducing it to an ordinary q-difference equation. The rest state
solution can be expressed in terms of a product of q-exponential functions in
the separated variables.
