I have a Leer also and looked at the same situation.
Even if they had a gutter the lift gate (back window)would have run off come off of the window as well, when opened.
What I do is I have a Thule mounted to the top and with a cheap tarp attacked to the Thule bar and draped over the Lift gate (window). I also have a large tarp with polls and stakes to tie it out for a awning. I use some of the black pipe insulation over the bottom of the lift gate to prevent rubbing of the tarp on the corners.
Frenchie
That's where most of your water is coming from, not off the shell itself (unless you're parked nose high maybe). On my last Tacoma shell (unknown brand) I glued a piece of rubber over the hinge, attaching it to the shell above and the window frame below using contact cement. It wasn't that noticeable but I still had water running off the sides, just not on my head or sleeping bag. The drip gutter, turned down to stop water running off the glass, would probably work better but you'd have to make it wider than the window to keep from having the same issue with water on the sides.
For stealth you could probably rig up some sort of waterproof material inside, under the hinge, that would catch the water and divert it to the side, just have to be creative.
