Transmission Trouble

I'm driving a new-to-me Ford Explorer XLT, 2WD, 4.0 6-cyl, with 201,000 miles on it. Engine runs quite smoothly, and I've had no problems with it until now.
The problem I'm having appears to be in the automatic transmission. It started out as a knock in the engine/transmission when changing between 1st & 2nd gear, both up and down.
Now, the transmission is taking too long to switch up to second gear. (No problem down-shifting as of yet.) I have to be up around 3,000 RPM for a couple of seconds before it changes, when before it would change just after 2,000 RPM.
There's more to it. Today I hit the "OD Off" switch a couple of times in case that made a difference. (OD is now back on.) Now the indicator light is flashing after about 15 minutes of driving. The manual simply indicates that I need to have my engine serviced, but there's got to be more of an explanation.
Here's the big catch: I just checked the level of my transmission fluid, and it appears to be like 1-2 inches above the regular level indicator. Could having too much transmission fluid cause these problems? If so, can I drain & refill to the proper level myself?
Thanks for your help.
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If it's new to you, the previous owner probably had the same problem and traded it in. Make sure you check the level with the engine running and the transmission in park.

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Rex, I checked the fluid once again with the engine running (in park) after driving it for about an hour. Engine was nice & hot.
The transmission fluid level was about 2 inches above the top of the normal operating range.
So could too much tranny fluid theoretically cause problems like this? I know I need to get it serviced, but I'm pretty sure I can drain the fluid myself if it's a problem BEFORE taking it to get serviced.
Thanks.
PS - I bought the truck from an old lady about 8 months ago, and the transmission problem has only started in the last few weeks.

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Zed wrote:

My guess it that it was run at too low a level and wore itself to where everything was loose - and they overcompensated by stuffing as much fluid in as possible.
IME, 50% of all used cars are sold because of transmission problems, and 30% are sold because of engine problems like bad gaskets or timing or whatnot.
It gets to be so common that I ask up front what is wrong with it.
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It's slipping.

The OD light means the processor sees a problem with the transmission and is telling you. The processor looks at transmission input speed and output speed. It also looks at how long it takes to make a gear change when it commands one. In a nut shell it is looking for slippage.

Too much fluid will cause damage just like not enough. You really need to have it repaired and soon, the longer you ride around with it doing what it's doing the more it will cost to repair, if it is repairable.

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