See if one leg is longer than the other. That was one thing that was checked with my pro bike fit. They shimmed one of my cleats to even out the slight difference.
One-legged cycling might help strengthen the hamstrings and increase their endurance.
Maybe some stretching after your rides?
If you're muscling it up a steep hill where you can barely move the pedals it should motivate you to use the hamstrings in order to get the pedals back to the one o'clock position.
Hmm, maybe PT exercises and weights don't sound so bad after all.