1991 Explorer not moving very well

I have a 1991 Ford Explorer xl 4x4, 225,000 miles, 4.0l engine automatic transmission. Today I exited off the interstate and when I tried to accelerate from the stoplight, the rpm's revved to about 4500 but the truck was crawling at about 5 mph. I stopped, put it in park and checked the fluid level, which read about an inch above the full mark. When I put it back in drive, it engaged and drove normally. I parked it downtown at a meter, and when I returned it started doing the same thing. I decided to move it to a better location so it wouldn't be towed, so I put it all the way in 1st and got it up to about 12 mph by revving it up hard. Two blocks later I let off the gas to slow for a parking lot and it went into first. I got it out of downtown in 1st and when I got near a neighborhood where it wouldn't be towed I tried to put it into D, where it started to drive normally, no shifting problems, nothing. I live about 15 miles outside town so when I got home, I put it into reverse to back into my driveway and it engaged normally, however when I got it in and tried to put it into drive it wouldnt go in again. I would like to note I haven't added any fluid recently, so I know I didn't overfill it on fluid, why does it read so high on the dipstick? The fluid is a normal red color, no debris in it, not burned, nothing. I am baffled, can you help?