why won't my car go into gear/ only go into gear when I rev it up to 2000 RPM's?

I dropped the pan, drained the transmission fluid for about an hour. I changed both filters, cleaned out the pan and replaced the gasket. Put 6 quarts of ATF+4 back into it. After I started it up and let it run for about five minutes it wouldn't go into gear. I put the Jeep into reverse and reved the engine slightly hoping to pump the fluid throughout the tranny and torque converter. Eventually it went into gear around 2000 RPM's, but was moving very slow for the amount of gas applied.