Hmmmm....Have you had an xray?? I know you've had an Wikipedia reference-linkMRI but this would only rule out muscle or tendon tear. An xray is also valuable to rule out calcium deposits in one of your Wikipedia reference-linkrotator cuff tendons or some bony abnormality around your shoulder which could be causing the impingement.

I think your leg/butt symptoms are likely to be related to your lower back - but the CAUSE of both shoulder and leg pain could be related.

To get on top of a shoulder impingement problem you need to strengthen your scapula stabilisers (shoulder blade) initially on their own and then work on strength exercises which incorporate your shoulder blade AND rotator cuff at the same time. You shoulder blade muscles are an important anchor to help take the load off your rotator cuff, but the timing and co-ordination of shoulder blade and rotator cuff together is important or else when you return to sport or activity you'll be repeating the same bad pattern. The other important muscle group is your "core" especially if you are wanting to return to body building. Just as the shoulder blade helps support the shoulder, your core helps support your entire upper limb activity so a lot of the force generation comes from good core strength - again taking some of the load off your shoulder. If your core is weak it can also cause lower back problems which puts pressure on your sciatic nerve which can cause butt/leg numbness. Is this all making sense. If I was you I would:
- get an xray of you shoulder
- see a physio to address your lower back problems
- get a good exercise progam which focuses on core strength, shoulder blade strength and rotator cuff strength starting with exercising these groups individually and then more complicated exercises which combine all of these muscle groups at once so you are co-ordinating using them all together.
- Taping your shoulder in to a better position can help reduce symptoms while you are waiting for your shoulder strength to build up. Taping can often help to reduce pain so that you can exercise better (your physio should be able to show you this)
- Stretching of your upper traps and your pec muscles is also important....especially your pec minor which is the smaller of the pec muscles (get your physio to show you) Check out my website if you need an exercise program