Well..update..repaired rear diff and re-shimmed. front and center diffs are fine. So ran it today and it still feels like it has a slipper clutch and the clutch is to loose. Under light acceleration, no probs., under hard..it seems to slip...
So, I am thinking something is wrong w/the motor.

Let me switch motors and see what happens.
Well, I switched motors and dropped in my 1512 1.5YF to see what would happen...ran like a top so it is definitely the motor.
1. MBX-6 T8 1900KV, RX8 ON 4S
2. MBX-5T 1520, MMM ON 5S
3. MBX-5 ONROAD CONVERSION 1515, MMM ON 5S
4. MRX-3 ON ROAD CONVERSION 1512, MMM ON 6S
5. TEN T 2650 T8, MMP ON 3S