Is it too late for a first time tranny flush at 48k miles...or should I drop the pan first and flush later?

If you have never done heavy towing with your truck I would just have it flushed. If you have +75k then I might be a bit more concerned.

It's hard to say, as you're right at that mark of almost having gone too long without a fluid change, but yet, you're not too far out.

I had my first fluid change out at 35k - pan dropped, new filter & 5 qts put in.

My first tranny flush came at 60k miles, I was very nervous, but it worked out alright.

I honestly don't know what to recommend, as I know these 4L30-E tranny's are very partial to fresh fluid every 30-35k, some even change them every 25k miles.