We have tried several approaches to mechanically thin peach, plum and nectarine flowers or fruit. These include trunk shakers to remove flowers or fruit and a drum shaker that has vibrating rods inserted into the canopy to remove developing fruit. These show promise, but the most consistent results have been obtained with a Darwin String Thinner. This machine, developed by a company in Germany and marketed in North America by N.M. Bartlett Inc, has a 10' rotating spindle with 2' long strings spaced every inch along the spindle. It is most effective during bloom and works particularly well on hedgerow or "V" shaped canopies that present a continuous fruiting wall down the row. Many factors such as spindle rotation speed, tractor speed, string type and string configuration can be altered to get the desired level of thinning (Darwin Thinner - 2010 CTFA Report). One model has a shorter spindle (about 8.5') that can be run horizontally over the tops of the trees. So far the Darwin String Thinner has not worked well on plums.
Penn State Website on Mechanical Thinning