Christer Weinigel wrote a series of three posts about how he built a active differential scope probe which is usable up to about a GHz:
This is part three of a series about building a active differential oscilloscope probe. Part 1 covered why I wanted a differential probe and the design of it. Part 2 covered actually building the the probe. The probe had much lower bandwidth than I had hoped for but it was still useful.
Project info at Christer’s blog.