This is just one possible build using grave injustice, and none of the skills are really mandatory (besides spirit walk). I'm sure there are a lot of other good ways to make this work, and I encourage you to experiment. Find what works for you.
Note: If you have some decent mana regen, or don't find mana to be much of a problem, one thing to consider is removing Firebomb. This lets you have an extra utility spell, and there are a lot of good ones available.
Here are a few skills you might try:
Specifically the Angry Chicken rune, because while you are a chicken you gain +15% movespeed. This effect isn't listed on the skill, but trust me, it's there. If you combine Angry Chicken with Stalker you can get bursts of +35% movement speed total. There is a video of this combination in action Mawt's guide below, check it out.
Rune of choice for higher Monster Power farming. Obviously you'll need to swap in Zombie Dogs as well, but it is worth the switch in order to pick up Sacrifice. You can use the Final Gift rune on dogs for some nice synergy with gruesome feast, and +mana rune (Pride) for sacrifice. Using Pride is very useful for filling up any time you run out of mana which is especially useful while running in groups or doing higher MP levels. The damage is also quite nice, not to mention the dogs home in for you making it effortless (and fun) to use.
Elites can sometimes take up a lot of time, especially with annoying affixes like shielding, extra health, or reflect damage. Big Bad Voodoo would help take those pesky elites down, and grave injustice would help with that insane cooldown. Slam Dance would be my choice, but if you don't have much mana regeneration then Rain Dance would work as well. Rain Dance is also a good choice for grouping since mana is a bigger problem in groups.
The nice thing about mass confusion is that it reduces some of the incoming damage, and also helps hold ranged elites in place when they would otherwise be annoying and run around. On top of that it increases your damage output by making some of the mobs help, and making you do 20% extra damage if you use Paranoia.
This can be taken instead of zombie bears, or even in addition to bears. I messed around with using both zombie bears and acid cloud for a while, mostly due to acid rain's ability to clear trash mobs extremely quickly. I'll sometimes find myself slowed down while I spam bears at everything, but acid rain can handle that sort of situation no problem. Eventually I switched to using ghost bomb instead (I was running no primary), but this is definitely a skill to consider.