The shoulder tension might be sorted out by relaxing more. When you're running, make sure you're not gritting your teeth, making white-knuckle fists or pumping your arms to drag you along - try to let them swing loosely and take up the rhythm that is natural to them.
The back pain is a harder one to crack. As a big lad, make sure you've been to a proper running shop and got advice on shoes. They'll likely be expensive, but it's worth it to not do damage to your body.
Other things you can try are to make sure you're not smashing each step into the pavement. Try not to push down into the ground to propel yourself. Make sure you loosen up and warm-up / cool-down at the start and end of your run. This just involves not setting out at full-pelt and then slowly jogging the last half mile or so. You might like to research some stretches you can do after a run while your muscles are still warm. Doing exercises such as planks and bridges will help build up your core stabiliser muscles: you're not just using your legs to absorb the force when you run.
Good luck!