I am facing the same problem, and when the chain is on the larger Chainring, it feels stiffer (grinding feel) when back pedalling. BB, freewheel is in good working condition. I am guessing if this problem could be related to chain length, because as I slightly push the rear derailleur to ease the tension of the RD spring (at the same time, the tension of the chain is eased), then no stiff and difficult when back peddling.