Assuming you meant md to diff, I would do <C-w>o to get back to one window and :Rex (short for :Rexplore) to go to the listing. (Don't use the window command, short for :only, if you have other windows you don't want to close!).
When you opened the folder using vim, vim thefolder
navigate to each file using arrow keys and press mf, it marks the files,
then press md, it calls vimdiff on the selected files (up
to 3 files).
There are other commands in this mode, use :h netrw to find more