Same symptoms on mine required a rear diff rebuild, added shims, and limiting up-travel of rear shocks.

After doing what you've already done:
I checked the outdrives to be loose, they were.
I added shims inside the diff to tighten, and that helped, but it was too late.
I found that Traxxas teflon bearing washer / shims can work for this.
I added as many shims as I could fit... (~2-3 each side), and it helped some, but it still slipped some.

Once I got new gears to rebuild with, I was able to compare and see the wear on the old gears (slightly rounded edges, hard to see without side by side comparison).

Also, need to limit the up-travel of the rear shocks to prevent overtravel allowing A-arms impacting the outdrives causing binding. Added spacers under shock boot.

Hope this helps.