A poor mans diff probe is to use two channels simultaneously and a math function.
If you're worried about accuracy this won't be nearly as good as a diff probe, though.
No contest on that
BUT.. with a bit of fiddling about getting both channel gains to match

and 'square as square can be' 10x probes calibration
one can get a lot of poor persons non critical tasks done in dual channel diff/invert/ADD mode,
with no wallet biting probe croc clips fitted or BANG potential earth/ground BNC/scope chassis connections necessary.
I've had good enough results doing this with 15 to 200mhz scopes, and on mains***
with extreme caution 
OP may get some serious bandwidth doing this with a 350mhz killa scope
Best to practice this stuff*** on very low AC and or DC voltages FIRST before every session,
and take physical notes of the setup for repeatability and or future use,
before diving in and over-driving the probes and or channels into oblivion with higher voltages,
..whilst the scope screen displays no signal, due to misadjusted or bumped controls =

= $ $ $ $

EDIT:
It would be handy to have some sort of knockup external box or easy internal mod (switchable) that can invert the signal on channel 4 on a four channel DSO,
so an impoverished user

can have two poor persons differential channels using four calibrated x10 probes
or is this pushing imagination and hope beyond DIY (+cheap n easy) feasabilty?

..assuming this comment made any sense