As noted, you don't want the rear LCAs to go over center, e.g. pointing up at the back. Bad juju there, running all kinds of angles out into the ether and pissing off the anti's.
Pinion angle is also important. 2-5 degrees down works for me. If your axle is cambered, make sure the pinion angle equals the one the camber was set at.
Is your thrust angle correct? .3 degrees was pretty f-ed up on my car based on back to back testing.
I assume you don't have a cambered axle, but have you checked the rear alignment? A little negative camber is your friend, and you also want some toe in. Any toe out is basically fatal to straight line traction.
Have you tried it with the rear bar off?
The TruTrac is a good differential, but not if you are picking up an inside wheel (especially on the non preloaded ones like the 8.8) A locker will fix the symptom, but not the problem. If you switch to that, when you finally fix the rear suspension you will end up with a pushy pig of a car and a ####ty differential (IMO). Been there, done that.
I have been fighting rear traction all year on my new car and I think (hope) finally got a handle on it at Blytheville and started getting over 1.1g accel on course pretty regularly. I am still working on maximizing the lateral grip while under power and will see if my latest changes do any good at Nashville.
It has been an adventure that involved slightly adjusting a lot of things (over time) and making sure the basics were right and stayed right after all of those adjustments.
One last thing, as you noted, some event sites suck in the launch area. And the same site won't always launch the same. I have had superhero grip at Peru in the past at launch, but ran events there this year where I could have spun the tires through all three gears before the lights. Even Toledo, which is my new favorite site in the universe with its great grip, was pretty slick on launch.
Hope all this helps,