would my 1989 Ford Bronco be losing power if the exhaust manifold has a crack in it? and it's making a whistling noise when trying to get up to speed?

The common Ford cracked manifold will not cause a loss of power unless you have the 300 cid 6 cylinder engine and the crack has progressed far enough as to negate the manifold hold downs, then there could be a power loss as well as a rough running engine. I would check the exhaust backpressure, that could be your whistle.