88 k5 blazer

I have an 88 k5 blazer with some th-700 tranny issues. Not an expert by any means. During a normal take off the trans will not shift from 1st until hi rpms are reached. Then it seems to not want to go to passing gear. If I get on it hard on take off, it seems to shift out smooth(still no downshift for passing). Have just recently change fluid and filter. Could this be a TV cable issue? Or worse?

Thanks
Mike

Have you checked the vacuum modulator?