Joel, from what i can see, i think the rear axle is extending extremely fast, hitting the end of the shock travel, rebounding and repeating all in about 5 to seven feet.
If you can stiffen extension, i would be tempted to go extremely stiff, maybe starting as tight as possible. Also i would drop the launch rpm if you can some. If your suspension has a high and short instant center, that might be part of the problem.