I have a differential signal(a +/- signal) to measure. Can I use a single ended probe to measure it and then just multiply whatever I read by 2 to get the actual amplitude?
Usually the scope's input is referenced to earth ground so if you put your probes across the + and - signals, you will be shorting - to GND.
You can use 2 channels on your scope to measure + and - signals with separate probes referenced to ground and then use the "math" function on your scope to subtract the two like everyone else is mentioning here.
Alternatively, you could use an isolation transformer if you only have 1 channel available on the scope.
Hope this helps.
You short to ground if you connect the probe ground to a hot point. You can do this measurement 'floating' i.e. with no probe ground connected - the scope will always be referencing ground because it's plugged into 120VAC. Two probes measuring two points with no ground connection is safe. Or you can add or subtract them to find the net voltage. Probes are usually AC coupled. You can find the offset before or after the measurement by referencing 'X' to a known ground point - with a single probe.