When you hit
d, an operator, you enter Operator-pending mode. Vim will then wait for a motion command to be entered. In other words, when you follow
<leader>j that's not happening in Normal mode...the mode for which you've mapped those keys.
For Operator-pending mode you need to use the
:omap family of mapping commands, so...
:onoremap <leader>j /
Fortunately, there's a mapping type,
:map, that encompasses both Operator-pending and Normal mode so you can kill two birds with one stone using
:noremap <leader>j /
However, this also includes Visual and Select modes so you need to be mindful of that. If that causes problems then just create two mappings (