I have a rescue crestie who had a minor case of MBD, so I might be able to help.
That sounds like a pretty severe case of MBD to me. Once the swelling gets to the leg and arm joints, then it can be fatal to the gecko. Is there any swelling or mushy spots on the jaw? This means your gecko is pretty far gone, I´m afraid. You said that you only dusted the mealworms on occasion? Part of the problem lies there. Without the CGD in the diet to provide necessary vitamins and calcium, occasionally dusted mealworms is not nearly enough for a crested. As well, mealworms are not great for cresties, as they are very difficult to digest. If you had fed CGD with a treat of dusted crickets or roaches maybe once or twice a week, then the gecko would have had sufficient calcium levels.
At this point, get your gecko´s calcium levels up by allowing them to lick some calcium WITH D3 (Unless yours has access to a UVB lamp, then use calcium WITHOUT D3). Allow the gecko to lap up a small amount every other day for about a week. (And by a very small amount, I mean maybe 3 or so good licks, and then put the animal away. If you can afford it, get a UVB lamp. I know, they are pricy, but it may drastically improve your gecko´s lethargy. As well, stop feeding insects entirely. Only provide the gecko with plenty of water and a bowl of CGD. Since yours seems to have a serious calcium deficiency, I would add a tiny amount of calcium with d3 (unless you get that UVB lamp) to the food, but only every few days. (Maybe 1/16th of a teaspoon, I´m talking small amounts here.) The CGD has proper calcium levels that you don´t want to throw off, but if your gecko is too weak to eat, it needs calcium desperately. As well, remove any tall branches that the gecko can fall from, and move it into a smaller tank if possible. (Like a 5 gallon or even a Kritter Keeper), just so you can monitor it better.
Good luck! I am wishing you the best with your gecko!