"in your stead" is grammartically correct, Hereunder is the comment:
"In your stead" may be archaic, but 'instead' seems imprecise. For eg, while "he will go in your stead" sounds archaic, "he will go instead" leaves open the question: instead of what? If you expand it to "he will go instead of you", it doesn't have quite the same meaning as the original - "in your stead" is a more like a stand-in, but "instead of you" suggests a replacement. Yes? No? Suggestions?