Wish I could give you some encouragement, but I can't really. I do have a question: Have you pulled the codes? (Mine showed a P0478 some time ago, but it cleared and never reappeared.)
The 3rd gear went out on our '96 Express 1500 at 110,278. Our daughter was out of town with it at the time, limped to a transmission shop, left the keys and rented a car. After they rebuilt it, we picked it up (2 weeks later). About 3 months later, it developed a shudder from 1st to 2nd. It was in the local branch of that shop for about two weeks. A month down the road, the (somewhat diminished) shudder remained, and it developed a hard shift between 2nd and 3rd. Back to the shop for two more weeks. Five months later, we took it back for a very hard shift into 2nd. It was there for three weeks. (In each instance, they told us they would be able to "get to it" in 2-3 days.) Having read a ton of reviews for shops in the area, the time in the shop seems to be par.
The hard shift became intermittent, and, oddly, could sometimes be cleared by cycling the ignition. (Could be an electronic issue.) It has since been shifting flawlessly several trips, so who knows? (Clutch debris? Actuator was in a bad mood?)
I'm telling you this so you won't count on a "one and done" scenario. I strongly suggest not putting all your eggs in one basket trip-planning wise--transmission issues or no.
Also, it may be helpful to recognize there is a difference between replacing transmission fluid (it does break down) and "flushing" the system (which doesn't seem wise with an older vehicle). If you choose to have it serviced, they may be able to tell you how much metal was on the magnet in the bottom of the pan--although what you actually do with that information is still a question.
I am a big fan of proactive maintenance/repair. With an old van, I walk a line between "do it now, under more or less controlled circumstances" and "if it ain't broke, don't fix it." Hearing something new or "wrong", has saved me lots of trouble. (My husband doesn't hear those things either, but has learned to trust my ear.)
Best wishes for the two of you on all your journeys!