1997 Jeep Grand Cherokee

check diff oil level, if constant with road speed possible wheel bearing , if you can alter the pitch of the noise with throttle position this is a noisy diff, also get drive line uni's checked as well.

mark (mhpautos)

check oil in differential, if low,add very HIGH quality lube w/inert additive(recommended)this will coat gears and stop noise(if not too bad)Remember always check all fluids and grease as needed or more!!! does your car have a carrier berring?