Update...

So I ordered a new 5mm shaft rotor for my motor, and a buddy gave me a Tekin SC4X 5.5 to try.
During the week, I tore the car down and cleaned it all up, replaced the diff fluids ( it had been about 2 months since the last diff servicing) and installed the Tekin and took it to the track for testing, ran a whole 6000mah battery through it, and everything was great.
Swapped the Castle in it, made about 3 laps and it was slowing down again...only this time I could actually hear the truck, since I was the only one on the track, and it sounded like I had a really loose slipper...wait a minute, this truck doesnt have a slipper!
I took the body off and could see the fronts ballooning on the straight away, but the rears were just along for the ride, picked it up and spun the rear tire and it made almost 2 revolutions before the other one started spinning.
I was like, WTF? I just refilled the diffs...guess I should have pulled the gears out and looked at them, the Losi Diff Gremlin struck again...
Here I am, 30 mins before the 1st qualifier and I got my rear clip off, and the diff out, luckily I already had a brand new diff cup, already sanded and insert glued in, as well as some new diff gears, put it all together and went out and TQ'd, set fastest lap and won.
Now the Castle has a spare rotor!!!