Trenbolone is relatively safe steroid all in all. There is some concern about kidney toxicity, but usually exaggerated. The beauty of trenbolone is that its one steroid that has it all : Its highly effective in its own, provides all lean gains which are fairly easy to maintain and isn't very prone to cause side-effects. Finaplix particularly provides you with a cheap source of trenbolone as well. The problem is making the cartridges into a sterile injectable or transdermal.
This is a direct quote from the paper that was being espoused in the post you highlighted above. I just got off of a tren cycle and the back pumps were like nothing I have ever felt before. I would also get hamstring cramps if I was not really careful. IMHO if your pain is extending all the way down the back of your legs that could be some sort of impingment of your Sciatic nerve which is the main nerve that runs from your spinal cord down your glutes where it splits somewhere mid-thigh or below. Not to say that you are not having kidney problems but kidney problems are generally pretty high up and can move from the back to the sides. If your worried get it checked by a doc. Good luck.