I dont think mixing prednisone with methadone is a problem, but I would just forget about the prednisone anyways, save it for something else. 80mg's of prednisone is not a lot either, Ive for sure been prescribed 40mg's per day before. The liver destroys a large amount of prednisone, which is why some doctors prescribe medrol, which I think works better. Also, Ive never got any euphoria from prednisone whatsoever, none that I noticed anyways.
I think you're getting pissed because your detoxing, it makes you the opposite of happy, which makes you mean. I would try to get a hold of some flexeril, soma, baclofen, a muscle relaxer for the leg problems. Advil or aleve might help to. It sounds like your muscles are tightening up if not spasming. Especially having to lay down with your legs straight, that sounds like hamstring tightness. Ive had the same kind of problems before but from physical problems and not WD.
Are you able to get more methadone so you can do a slower taper?