Transmission issues

My '99 Explorer 4x4 4.0 V-6 got the opposite problem a few months ago. Trans, overdrive, and 4 wheel drive continued as before but reverse gear was same as neutral - except on level ground it would creep a couple of feet backward sometimes. I just got used to only parking facing uphill, so I could let it roll backwards in neutral when I left.

After a month or so of that, I was slowing down for a right turn, when at the moment when it usually shifted to low (10mph), I heard a sound like a wheel bearing grinding, so I put it into neutral, and coasted to a stop at the right shoulder. I looked underneath at the U-joints, etc - saw nothing wrong, so I started it back up and put it into drive. Nothing happened. But when I put it into "2" (second gear?), it did fine.

At this point, I had 3 neutrals: "R", "N", and "D". Then I tried "L". Nothing happened (the car didn't move). That is 4 neutrals, a "Park" (P), and a "Go" (2). A few days later, I discovered that "D" (drive) works fine if I shift from "2" to "D" only after I am going 20 mph or more. It even goes into OD on the freeway. But if I go under 15 or so mph, it shifts into neutral. That's fine if I am stopping, but I have to shift to "2" again when the light turns green.

Does anyone have any idea what I should check or replace, and whether this problem will get worse? The Explorer has 225,000 miles, and is on its second motor. Long before losing my reverse gear, I noticed that it freewheels (coasts) downhill in "L" but not in second. Does that mean it has a bad carrier one way clutch or a loose reverse band? I understand that there are 4 solenoids in or on the transmission. Should I have the fluid and filter replaced, and look for a bad solenoid? Could it be the torque converter?