I don't think the oscillator on a PIC would be good to 2ppm absolute accuracy even with a very good xtal, unless you FIRST calibrate it against something that has already been calibrated, therefore it doesn't get you far. It will however be good enough to calibrate your scope since that would only need 1% or so, and any old crystal should achieve that, even with a fairly primitive oscillator. For frequency calibration, your best bet is to receive an off-air standard, for example GPS or in many countries there are low frequency standard transmissions (50kHz, 60kHz, 77.5kHz or others, look up which ones are available in your country). It is quite feasible to build your own receiver for these. These transmitters are maintained to a higher accuracy than any piece of hardware that a hobbyist could afford (e.g. 2 parts in 10^12).
Chris