When listing diff fluid weights it's common practice to list front-center-rear. X-X-1/2x is just a guide since the rear is usually significantly lower in weight than the others, generally half (or sometimes less than half) what the front is.
Your first task should be to increase the weight of the fluid in the center diff. The heavier the fluid, the more evenly power is sent to each output. The lighter the fluid the more likely an output will "unload" and receive more power. Under acceleration the vehicle weight transfers to the rear planting the rear tires. The front have less traction, and start spinning, without a heavy enough diff fluid to "lock" the front and rear together, the fronts spin alot faster than the rear and wear out the tires. Heavier fluid will help prevent this, increasing acceleration.
Here's a guide to 3 diff tuning that'll help you along
http://www.rc-monster.com/forum/showthread.php?t=5050