Why not build an instrumentation amplifier? With a simple 3 op-amp circuit you get very high input impedance, very high CMRR, and you can even build in all the gain you want. At 1 MHz this is easy, you could even breadboard it with decent results. This all assumes that the "few volts" of DC offset still keeps the signals within the rails of the op-amps. Another option is a special purpose high side current sense amplifier IC.