Lifted ground in serial communication?
Unidirectional links are easy to isolate. Bidirecional links, a little harder.

A serial interface (RXD, TXD) is very easy to isolate: two simple optocouplers, plus assorted resistors, and you're done.

If it is I2C, then the bidirectional DATA wire will need some special care. There are specialized I2C isolator chips for this purpose. Here is an example (I did not read it in detail):

http://www.nxp.com/documents/application_note/AN10364.pdf