I found out why my diff wouldn't spin smoothly. One of the diff bearings wasn't fully seated on the diff and was a little crooked so every half turn it was stiff. Replaced the bearing and clean the seat then drove it home firmly and squarely. Diff is much smoother now.
After discovering front diff issues I thought I better check the rear. One off the diff bearings was completely seized and had started to melt the diff case!
The original bearings have been replaced now with new aftermarket ones and I replaced the diff case.
Looking forwarded to getting it back out on the track.
Check your bearings folks, these big 4 pole motors will power through the friction but the car suffers.