id have to disagree with big mike respecfully
i have broke a 21 spline input on the back shaft and seen a couple others.
that said ,it is ok to run but keep it in mind the increase of torq and gearing that the shaft wasnt designed for
if it is a stock input shaft.
id say you need to be aware of wheel speed and HP when in low range with the front case and in 4X4 HI in the back
