I'm kind of with FB on this one. But...since the problem manifests while climbing in the lowest gear, why not just take the brakes completely off the bike and ride it up a steep hill. (Just be sure to walk the bike back down the hill). If the symptoms go away with the brakes, then you're on to something. If you still feel resistance, then at least you've eliminated the brakes as its cause. Either outcome puts you closer to a solution.