Differential Pairs and Power Planes
First of all, high speed signals don't care if the plane is power or return (GND).  A trace carrying a high speed signal will couple to either or both of them, depending on the distance (height) from the trace to the plane(s).
That being the case, you want to avoid running high speed traces, across splits in planes.  If you absolutely have to do that, then you want to place capacitors across the gaps in the planes, near where the trace crosses the gap.
Differential signals are somewhat more forgiving, but you still want to follow the same rules for routing high speed single ended signals.