so this is my first post. please give as much advice as you can!

I've had a mountain bike for 4 years. I ride it most always in the city or on well worn trails, a lot of commuting, a lot of biking in the rain cuz its Portland, OR. Due to upper back pain, I am considering
a) getting a full body fit to my bike (I just learned about this option)
or
b) purchasing a hybrid

Here are my questions:
1. is is true that hybrids are lighter than mountain bikes?
2. are hybrids similar enough to mountain bikes that I should just get a fit to my current bike and move on with my life?
3. if hybrids are significantly different from mountain bikes then can anyone reccomend a good one to look at?

thanks--