Chris id suggest going up on the diff oil.
With the bigger motors the diffs take a lot of punishment especially the centre and unlike a .21 powered truggy the need the thicker oil
Id suggest 20k.
When he ran his truggy was teh front wheels ballooning a lot, that would also confirm that the center diff oild is too thin.
Have seen this happen on a few Mugen and X1CRT truggies.
Also make sure there is no drag on the driveline and the brakes are not binding.