[Al Skierkiewicz]

Chris,
I went to the site Mike linked to and wow, you get a lot for little money. It looks like it would solve the problems with using a sound card. Cables look a little short but you can probably get around that.
When sensors just are not working right, a scope with better that 15kHz bandwidth can tell you a lot. Some failure modes produce a lot of noise in the output and a low bandwidth may attenuate those spurious signals to a point where you would think they are insignificant. It is also possible that the sensors can't keep up with the stimulus, and a scope will show you the effect on the sensor output as speed increases. From and electrical standpoint, I will grab my Fluke first and a scope second.
As to using transformers...the device will couple signals without having to tie chassis to the circuit ground but it does so with magnetic fields. A shield is required (transformers are available with shields) to prevent magnetic fields from coupling into the transformer. Spinning motors do have some nasty magnetic coupling that would affect the display and accuracy. One other thing that came to me is that sound cards are capacitively coupled so looking at a DC component is out of the question with a sound card solution.
Many teams do not carry scopes with them to competition. We usually carry a beat up one and actually take it out from time to time to check on something. Since it is not new or in good condition, a bunch of bubble wrap will usually do OK to put it in the shipping container with the robot.