I have a 2001 ST that I bought as a non-runner over 3 years ago. It had 184K on it. As far as I could tell the truck received little-to-no maintenance over it's lifetime.
To focus on the transmission, my fluid was black. It shifted perfectly and still does 15K+ later. All I did to it was a pan drop and filter change. After replacing 5 qts of Mercon V. The fluid is still very dark, but it still shifts perfectly. I had added a drain plug to the trans pan and planned on doing additional fluid changes, but due to a host of other issues with my truck I've never bothered.
I've read that the transmissions in the V6 Explorer trucks (the 5R55) are not the most reliable, but I figure with the abuse my truck suffered over its previous life I must have a pretty good one. The one issue I have is that if heavily stressed in reverse my band slips. I was told that this is likely being caused by a weak EPC solenoid because the reverse band/drum is very heavy duty (the EPC solenoid is a $40 part and it is a fairly easily DIY replaced part, but I haven't bothered with that either). I only use my ST around town and for trips to the dump.
I think the biggest thing that kills these transmissions (any automatic transmission) is heat, even with the OE trans cooler. I believe Ford recommends fluid and filter changes at 30K intervals.
In an over-abundance of caution, you could replace the valve body and solenoids, but with only 136K and that these are repairs that can be made with the transmission in the vehicle, I don't see the value in tearing it apart unless there's something wrong with it.
BTW - I do not recommend you have a reverse trans fluid flush done on your vehicle. If you want to get all the fluid changed (trans and torque converter) do it with 5 gal buckets through the trans cooler lines.
