Transmission question P0734

[RFaber]

Hello all,
Recently I had my transmission failsafe light come on.
here is the circumstances.
We were hauling our travel trailer (3100 lbs. as measured on DOT scales)
our X5 is the 3.0i with the GM A4S-270 transmission. (I believe!)
I have not changed fluid in it, (come to think of it, neither did the previous owner if I recall correctly) the Truck has 280,000km and until then ran flawlessly. I have a scan tool, as well as a laptop that has INPA and a bunch of others (Romanian software)
While we were driving, I needed to merge in with traffic, I was travelling approx. 80km/h (50MPH) and punched it to get into traffic, my truck stuttered, and then the dreaded transmission gear light came on. I pulled out at the next stop, about 2 miles down the road, and plugged in my scan tool in and found I had error P0734. Incorrect Gear ratio detected 4th gear.
after sweating for a bit, I reset my transmission light with my scan tool , said a little prayer, and continued on with absolutely NO MORE issues.
now, my question is, Did I do anything wrong?? should I change transmission filter and fluid? or at least, check to see it its topped up!?
I have no leaks anywhere, and the truck didn't seem hot, and shifted properly and without any issues. although, I was a little 'lighter' on the throttle! when we got to our destination (Seaside Oregon) we offloaded our trailer, and drove the X5 around with no problems or issues.
So,
Should I change fluid? should I top it up? should I change the transmission filter, should I do nothing!?
I've read several other posts (thanx RRPhil and JCL!) but I haven't had a good clear answer! I gather, the input speed sensor and the output speed sensor differed enough in their readings to give me an error, indicating something slipped, so it went into fail safe, I drove it easy, no slipping or anything wonky in the feel of the drive.
I just need my mind set at either, ease, or resignation that I need to do something with my transmission! (is an external cooler advisable!?)
and is there any way besides a scan tool to read the transmission temp!? (i.e. on the OBC menus etc.??
Thanks all!
Riemer

[pearlpower]

At 173k miles, your fluid has done it's job, and is in need of some care. Recommend a drain, filter change, and fluid. Not a flush. If nothing abnormal occurs in a few thousand miles, I would perform another drain, and top off. Again, not a flush.

[RFaber]

Thanx, that's what I was thinking! Thanx, just concerned if I did any damage! I guess I will know once I pull the pan!