I had raised my rear diff on my 1969 firebird with a floor jack to check if it was a limited slip or open. Upon doing so when I spun one wheel the other didn't move at all (and vice-versa). While spinning it I had a friend go try to spin other wheel in opposite direction to see if there was any resistance, there was not. But if one side would start spinning faster then the other ujoint would start spinning the opposite direction. Is this normal on older cars or what could the problem be if so?