You could try the medium plastic parts (where not used).
Try without the front wing and move the rear wing to the high position.
Try 4 degs toe in.
What wheels are you running - do you have the correct offset on the rear if not running X-Ray wheels?
Try mounting the rear shocks onto the rear (you'll need to cut the wing though). Either inside or middle hole on wishbone.
Can you advise what tyres and motor you are running, whether you have any boost/turbo on the speedo? Whats the pile of the astro like? Much sand?
Tyres make a massive difference, especially when trying to get a car designed for high grip to work in medium/low grip. Do you have the same problem on new tyres?
As already mentioned, the under lipo weight can help calm things down.
How did you arrive at the 5k diff oil? I've found that if the grip isn't high enough, the gear diff can make the rear lively under power, particularly if there is a lot of lateral weight transfer (like the inside wheel goes light and the rear pivots much more than desired). So ball diff might be an option to look at if you are still struggling.
HTH
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