Bob,
Better pick up a couple quarts of whatever fluid you need (I think the Ally takes a different fluid than the GM standard Dexron VI)so you can keep 'er topped off while you decide what to do.
Yes, cooling lines on the GM vehicles have been a problem just like the brake lines and fuel lines. For some unknown reason they put in a stainless exhaust system but used uncoated steel for the brake, fuel, and trans cooler lines.
I would look for a transmission shop that has experience with the Allison units, not necessarily a dealer. Unless your unit is really puking fluid, you should have some time to find the right place and get the job done right.
If they need to pull the trans to complete this repair, this may be a good time to look at a rebuild. 90% of the cost of a trans rebuild is the labor not the parts. If you're going to pay to have the trans removed, may as well change the other parts that are worn and likely to fail also. I wouldn't give up on the truck, nothing lasts forever, just needs a little TLC is all.
