Hit it harder.
Seems like lame advice, but it's probably not in your case. If you have hit enough of these shots, and are able to analyze your weakness to the extent that you can say with a reasonable degree of confidence that 'your slice lob is usually short when you're on a dead run,' then you are able to make simple corrections to your game.
The BH lob-on-the-run isn't an offensive shot by any means, and if you're being forced to hit one, you're all but out of the point already. The best you can hope for is to neutralize this bad situation as often as possible. You're not trying to hit an offensive shot; you're trying to hit a defensive shot that will succeed the greatest % of the times you try it over your tennis career.
That said, A) you know that right now you 'usually' hit it too short. Usually, as in the majority of the time. This is no way to maximize your results. B) You and virtually everybody has the strength to hit that ball over the back fence if you put your mind to it. You throw it up short because you're scared of hitting long. Not because you can't hit it deeper.
So you have to get over that fear and start hitting harder. It'll go deeper. It'll take a while before you figure out how hard is hard enough. And when you do, yeah, some'll go long. But if you have decent feedback mechanisms working in your brain, you'll get it to where the % of decent defensive balls hit that way is at a higher % that your current sub-50% number. Just get past the don't-hit-it-long psychological hurdle first. Just remember, hitting it long isn't that bad when the alternative is setting the opponent up to feed you a fuzz sandwich.