jjmg53,
I have developed my own spreadsheets for external stability (handles superimposed walls, backslopes, foreslopes, surcharge loads - everything but limit equilibrium slope stability) using the FHWA design guidance (FHWA NHI-00-043). It shouldn't be too hard to develop one yourself for internal stability.
This has the added advantage that you have a better idea of what the limitations of the spreadsheet are!
Jeff