  The magnetic flux distribution is determined by the solution of the
Grad-Shafranov equation. With differential rotation, i.e. the variation of the
iso-rotation parameter, the shape of the light surface must be calculated in an
iterative way. For the first time, we have calculated the force-free magnetic
structure of truly two-dimensional, relativistic jets, anchored in a
differentially rotating disk. Such an approach allows for a direct connection
between parameters of the central source (mass, rotation) and the extension of
the radio jet. We present an analytical estimate for the jet opening angle
along the asymptotic branches of the light surface. In general, differential
rotation of the iso-rotation parameter leads to an increase of the jet opening
angle. Comparison to the M87 jet shows agreement in the collimation distance.
We derive a light cylinder radius of the M87 jet of 50 Schwarzschild radii.
