97 Explorer Auto Transmission

I have recently noticed that the transmission take up to 10 seconds to engage, when shifting from Reverse to Drive. Sometimes it doesn't take as long.
I haven't noticed any other problems, it appears to fine otherwise.

After hearing all the problems with these, I want to take care of it before it becomes a huge problems. Mileage is 120,000 and it has not been changed since I have had it (about 50,000 miles).

I plan to replace the filter and fluid this weekend

Any suggestions?