Hi
The "ground" is the problem. The switcher is likely operating at a high enough frequency that a two probe A minus probe B differential setup will be a challenge. The probes you show are *safer* for a high voltage circuit, but they are not *safe* in an absolute sense. Anything that comes along the mains feed will hit those probes. It is not unheard of to see crazy spikes.
Other ways to do this:
1) Get a nice big isolation transformer with all the proper leakage and shielding specs on it. Invite a large friend over to carry it to your bench for you
This will let you feed one side of the supply with a grounded feed. Still not ideal, but not quite as crazy.
2) Get a proper differential probe with high voltage certification. These probes have an external (analog) difference amplifier and proper protection. You can buy quite a few 1054Z scopes for the price of one of these beasts.
3) Consider what you are trying to look at. Is all the "stuff" you want to see above 10 KHz? If so, a couple of high voltage (> 2KV) capacitors may be what you want to get first. That gets rid of the nasty mains voltage (high pass filter) and gets things down to (maybe) op amp territory.
Lots of things to think about.
Bob