Hi Tacitus,
Your post interested me because I have had a lot of weakness and what I call shaking. Mine was in large part from tetanus, though prior to that when I was under a lot of stress and I held my hand out in front of me it shook quite noticeably.
The thing with tetanus is that it's a central nervous system disease, so although I felt as if my muscles were the problem I knew that it was really my nerves.
I used a lot of vitamin B12 injections until I discovered (through Fred who used to post on this forum or one like it) that Methylcobalamin works MUCH better than cyanocobalamin when comparing the two forms of vitamin B12 in their lozenge form.
Now I use only the Methylcobalamin lozenges and as long as I remember to use a lot more when there is increased stress I don't have the problem, and my handwriting is VASTLY improved.
Now, if the problem you have is related to low B12, then it could be because the mechanism in your digestive system that gets it out of food for you is disrupted or in some way malfunctioning. If that's the case, then you may also be low on magnesium which is gotten out of food by our bodies in nearly as complicated a way as is B12.
I used to have a lot of leg and foot cramps, but since I've added magnesium to my daily vitamin and mineral supplements I haven't had the problem AT ALL.
I hope things improve for you.