I own a 91 explorer 4x4, and when I driveover 15 mph, a very steady whining noise comes from the car, and when coasting the noise decreases, what could this be?

Starting simple. Tire noise?

and when coasting the noise decreases, what could this be?

if letting the strain off the drivetrain decreases the noise, you might want to top off the fluids in the axle's. then inspect the uni's on the driveshaft. Whine indicates metal to metal, and it sounds like it is most likely in the axle. fill or change the fluid in it and try again. if this doesnt cure it it IS a bearing.