Transmission

Very doubtful the two things are related. If the mechanic says the transmission needs to be rebuilt you might want to consider trading for a safer vehicle. There is a big problem with rear axles on the Windstar and a recall is in effect.

Normally a gear ratio fault code means there's slippage taking place in one of the clutch packs in the transmission. That means it needs to be rebuilt. My numbers here are not accurate but to illustrate a point, lets suppose you were offered $5000.00 in trade-in value toward a different van when there was nothing wrong with yours. Now that yours needs $3000.00 worth of repairs, that does not mean they will only give you $2000.00 in trade. You will likely still get perhaps $4500.00. The point is you're almost always better off trading when repairs are needed, (because they expect repairs to be needed and have already factored that into what they offered you), and you are better off not paying for any repairs in an attempt to make the vehicle worth more.