If you suspect the battery, use a know good battery say your pickup or car and jump it. If you hear it spinning but not engaging it could be a bad Bendix or weak battery not spinning it fast enough to engage the Bendix. Assume this starter uses the Bendix type starter.
I am with TxJim, sounds like bad battery cable connections. The clamps on the cables can be bad where the cable enters the clamps and also the inside of the clamps.