I would not run anything less then a 13 on the front. Tough on chains. I dont think its too much tire, especially with the 4mil. I have same tire 22x11x8 10pdl SLS combo on stock stroke and works fine. Sometimes I have the same prob you explained, try playing with tire pressure. Little will go along way. Pump up to get more wheel spin off the line, too low and youll fold tires over and get mega traction or bog. Wet/dry sand will also mess with your launchs. I run 13-44 light drag and treeshots...
check this pic below, if you zoom in on the rear you can see im underinflated therefore tires are folding and get same characteristics your describing.