Ms - started rebif after a severe allergic reaction to copaxone (glatiramer). Now extremely itchy. Could it be the rebif or the MS itch?

Could be side fx. Rebif and copaxone (glatiramer) are not the same, but your symptom of itching could be caused by your new medicine. If it does not subside soon, say 2-3 days, it may be helpful to make a change, or use some kind of treatment for the possible side effects.
Yes but.... Itching has been reported as a sensory symptom in ms (uncommonly) but the only way to be sure that it is not due to Rebif is to stop the injections for a week or so and then restart. If the itching went away and came back with the injections you have your answer. If that happens may be you could tolerate a smaller dose of beta interferon in weekly injections of avonex (interferon beta 1a).
Allergy? You may indeed be allergic to interferons, such as Rebif, and you need to contact your physician. Really, no need to inject and get these reactions, as three oral medications are available and two of these are more effective than either of your prior meds.