Am Overweight. Pain In Shoulder When Exercising. Help

Question: I had always been fairly thin and healthy looking. After a serious lifestlyle change in my late teens I ended up gaining almost 50 lbs. I got sick of it after 6 months, bought a home gym and lost all the weight. I actually became a fitness trainer in my 20s. I continued exercising/lifting on and off and managed to stay relatively fit but occasionally would gain and lose a few pounds here and there. I even ran a marathon a few years ago. I'm almost 40 now and have found myself probably 35+ lbs overweight again. Still strong, with a lot of muscle I have managed to maintain somehow without lifting regularly, like I used to, but you really can't tell due to all the body fat. I don't know if it's age or something else, been to every doctor I can think of, but I am having some severe pain in my shoulder/arm and leg (same side) when I exercise that is keeping me from pushing myself as hard as I need to to make progress. It is actually stunting any progress and has somehow seemed to have a reverse affect due to maybe a discouragement or depression of sorts from it. I have always loved the pain of the pump and the soreness from healing muscle fibers, but this is different. I have gone to sports medicine therapists, gotten MRIs, gone to chiropractors, gotten X-rays, had nerve tests and anything else I could think of and no one can find anything wrong. I know my body... as anyone who has lifted for 20 years does and is a little more in tune than someone who hasn't. I have never known what it feels like to not be able to just go all out and get the workout done. One of my favorite exercises used to be incline push ups, feet off of a chair, with hands on the floor close enough to almost form a XXXXXXX so I could target the top inside parts of my pecs and my shoulders. When I try this now, one arm could go forever and the other is burning in excruciating pain by the fourth or fifth rep. It's a XXXXXXX burning pain, when I stretch it feels like a rubber band about to break, that sometimes feels like a bruised bone a pinched nerve or maybe the kind of pain you feel when you bend a limb the wrong way. Up to this point, I have been unable to describe it satisfactorily so that any of the several experts I have seen would say XXXXXXX oh yes, that is ______ and we can do X to fix it XXXXXXX The reason I am spilling all this is really out of desperation more than anything, but the short term end goal for me is that I am doing the Tough Mudder in October and am through my first week of Insanity and want to do one round of Asylum right after that and right before the competition. I am cautiously optimistic that I will be able to do this with this huge thorn in my flesh and unexplained and uninvited hurdle to my progress, but I can't say that it doesn't feel a bit debilitating at times. I'm sure many of you have accomplished more with much greater challenges, but this one is mine... and I need to beat it.
If you made it this far... thanks for reading. I anticipate the end of this story to be one of triumph and inspiration, but right now... I just need some of my own.

Hi, thank you for your response. I wish it was just a vitamin deficiency, but as I said I know my body well and there is something very wrong. This has been going on for possibly three years or more and has gotten progressively worse and worse. There is a dull ache in my shoulder and thigh/ glute at all times, sometimes making it difficult to sleep, and if I sit a certain way for an extended period of time my whole left thigh/glue goes numb. When I stretch these parts of my body the pain is much more intense and burns/ pulls in such a way that I can only describe as a rubber band being stretched the three point of almost snapping. It's not muscle pain or muscle cramps, these types of pains I know well from my years of lifting. It could be PN as you suggested but the doctor ran a nerve test with electric sensors and found no damage. Please help.