Interesting question. ENVI's shaded relief product is essentially Lambertian reflectance, calculated by taking the cosine of the incidence angle: cos(i) = cos(90-a)*cos(b) + sin(90-a)*sin(b)*cos(c-d) where i=incidence angle (angle between solar rays and the surface normal), a=solar elevation angle, b=slope, c=solar azimuth angle, d=aspect. You have four unknowns in this equation, of which two (elevation and azimuth) should be fairly easy to obtain by tracking down some metadata for your shaded relief image. That leaves you with two (slope and aspect) that are directly related to surface elevation. At this point, you would most likely have to build a program in IDL that could iteratively solve for reasonable slope and aspect values on a per pixel basis. I don't think that this would be an easy process.