Your clutches might be fine. Which direction are you trying to do the donuts in, to the left? or to the right? The rotation of the driveshaft twists the rearend to the left, taking weight off the right rear, and this is why most non-posi cars will spin the right rear tire. If you removed your sway bar, when you go to whip a donut to the right, it's allowing more body roll and taking even more weight off that tire, and it will overcome the posi very easily, even with perfectly good clutches. Try doing a donut to the left if you aren't already. I'm not saying your clutches are not gone, but if it will spin a nice posi in a straight line, this is most likely what's happening. If you want it to spin posi all the time, get a detroit locker or a good aftermarket posi unit.