GQgeek, having one shoulder lower than the other is a result of stretched ligaments and usually shows up on the dominant side (i.e. the right shoulder is lower for right-handers). I don't think it can be corrected by weightlifting, but perhaps by stretching (overstretching) the shoulder ligaments on the other side (
joking, this is not something you want to do). Having shoulders pitched forward is, however, something that can be corrected, but it takes dedication, hard work, and I would recommend the help of a medical professional.
The basic strategy for the shoulders is to strengthen the muscles that hold them back (the trapesius and rhomboid muscles), stretch the opposing muscles (the pectorals) and train yourself to sit and walk with shoulders held back *and down*. And when I say train yourself, I mean that eventually the posture with shoulders back should be your relaxed, most comfortable posture, achieved without effort.
I'm not a medical doctor, so how do I know this? Because I have similar issues and I'm working on them with a very good osteopath. Here's what I've done:
Since I'd been developing various pains for a number of years (low back, right hip, under the right shoulder blade) and they were getting progressively worse, I saw an osteopath in June 2005, on the recommendation of two different friends who were treated by him. He took X-rays to show me the asymmetries of my skeletal structure, including shoulders rounded forward. He had me fitted for orthotics (shoe inserts) to level my hips and began manipulations much like a chiropractor (except unlike a chiropractor, an osteopath is an MD). He also prescribed daily stretching and, later, weightlifting. After weekly manipulations and 1 to 1 1/2 hours of daily stretching for about 3 months, I began to see miraculous progress (straightening of the lower spine, no hip pain, etc.). Then he started me on a weightlifting program, primarily to strengthen my back muscles. After slightly more than a year, my lower back is straighter than it's been since I was about 13 (I'm now 45), my hip pain is gone, my shoulders are less rounded, the pain under my right shoulderblade is usually gone and when it recurs I have a stretch that gives me relief. I still have low back pain, primarily because I'm aggressively stretching my very tight lower back and hips. I've made tremendous progress, but I still have a long way to go before my shoulders (and upper back) are straightened.
So if you want to start down this road, I recommend you find a *good* medical professional such as an osteopath (Be forewarned that osteopaths are not held in high regard by the rest of the medical profession, but I consider them just another specialist. In contrast, two GPs whom I told about my pains were totally useless). I expect that I got lucky when I found my osteopath and you may need to visit 5-6 doctors before finding one who can help. Then take responsibility for your own rehabilitation (because that's what it is; I'm rehabbing from 25 years of sitting at a desk) and recognize that it may take years of stretching and exercise to get where you want to be. Since you're younger than I am, I expect the process will be easier and quicker for you, though.
For me, even though I studied martial arts for 8 years and consequently know how to stretch, and stretch aggressively without hurting myself, it may take me several more years of vigorous stretching and daily weightlifting (currently a 5 day split) to adjust my posture. In fact, I consider it a lifestyle change and a commitment I've made for the rest of my life. But I'll also tell you, I took care of my mother for the last 14 months of her life and I *do not* want to walk hunched over, unable to straighten up, as she did. On the positive side, I can now control muscles in my trunk that I never knew existed a year ago, I look better, feel better, feel better about myself, and I've gained 10 lbs. since I started weightlifting (and at 6'3" and now 170 lbs., I'm the very definition of a hardgainer). I only wish I'd done this 20 years ago; I'm convinced it would have been much easier.
I know this is more than you ever wanted to know about me,
but I hope the information is helpful. Good luck!!