I think you're actually looking for the Topographic Ruggedness Index, which is another tool in the SAGA Terrain Morphometry toolbox. You'd run it with a 3 cell window to mimic the Arc output. 

More here - https://gis.stackexchange.com/questions/6056/how-to-calculate-topographic-ruggedness-index-in-arcgis-desktop

edit: nope, wrong again. I've been poking through the Grid toolboxes but can't find anything useful, hopefully someone else more familiar with the tools can help. What you want is easy in R, though:

    library(raster)

    r <- raster('path\\to\\your\\file.ext')

    winmin <- function(x) min(x, na.rm = T)
    winmax <- function(x) max(x, na.rm = T)

    focalmin <- focal(r, w = matrix(1, ncol = 3, nrow = 3), winmin)
    focalmax <- focal(r, w = matrix(1, ncol = 3, nrow = 3), winmax)
    rel_el <- abs(focalmin - focalmax)
    writeRaster(rel_el, file='path\\and\\output\\filename.ext')

you'll just lose a ring of cells around the edge of the raster.