Frequency response across component in ltSpice
There are plenty of ways. You can use a voltage-controlled voltage source (an \$E\$ element) with a gain of 1, connect the (+) and (-) pins across your floating load with the correct polarity and tie the output to GND then probe the output of the \$E\$ -source. See below if you would like to plot or probe the voltage across \$R_1\$ for instance and reference it to ground for further usage in your circuit:



You could also use an analog behavioral source or a \$G\$ source feeding a resistance referenced to ground for instance.