How can I view USB traffic on an oscilloscope?
Don't ground the data lines. They are above ground. Beyond Logic says


  USB uses a differential transmission pair for data. This is encoded
  using NRZI and is bit stuffed to ensure adequate transitions in the
  data stream. On low and full speed devices, a differential ‘1’ is
  transmitted by pulling D+ over 2.8V with a 15K ohm resistor pulled to
  ground and D- under 0.3V with a 1.5K ohm resistor pulled to 3.6V. A
  differential ‘0’ on the other hand is a D- greater than 2.8V and a D+
  less than 0.3V with the same appropriate pull down/up resistors.


You need to connect scope ground to USB ground. Otherwise you're killing one of the signals and the differential receiver will not detect the required phase reversal.

With scope connected to ground a single channel scope can monitor either D+ or D-. A dual channel scope can monitor both and you should see the phase inversion when data is transmitted.