Lost second gear

Check fluid level and make sure you didn't overfill it, which can cause air bubbles in the fluid and loss of pressure. If this is a high-mileage unit, and hadn't been serviced in a while, changing fluid may have dislodged a lot of gunk which is now restricting the new filter. Take a look, it may need a second flush and filter. Then again, you have the older 700r4, with the governor (measured road speed) and TV cable (for throttle input). The TV cable controls line pressure to the valve body/hydraulic components of the transmission and an incorrect adjustment can affect shift behavior. It can also cause clutch plates to wear or burn out if misadjusted, just as the governor can if also not adjusted. Then again you might just have worn clutch plates.