The OP says "When I lift the bike off the ground by the seat, the pedals will go forward."...so presumably the rear wheel is spinning (forward). So why wouldn't the same happend when the wheel is on the ground? Just wondering if it's spinning 'freely' or if it might be rubbing on a brake pad...AND in the highest gear...the combination of which might make it appear to be seize or otherwise unable to spin.
Dan