I fixed the transmission coolant leak with some 3/8" hydraulic hose. Thanks for the advice HeavyOPP and JSwanson! I used a pipe cutter to cut out the leaky section of the OEM hard-line, slipping the 3/8" hose over each of the cut ends and connecting it with hose clamps. After reading up on this fix, I decided to use a pipe cutter to ensure the cuts were clean and wouldn't introduce metal shavings into the transmission lines. Thank-goodness people share advice in forums like this one! This splice leaves all the factory connections in place until a more comprehensive overhaul later (at least one other connection isn't actively leaking but looks dicey). The below-photo shows the line as seen from below. Before diagnosing the leak, I had removed the air-box and electric aux-fan for ease of access.
This leak makes me realize the importance of packing various spare hydraulic hoses, etc. for field fixes during any extended trip. I'm going to start organizing a ready-to-go kit of spares, fluids, etc. As I've learned to work on the Jeep, I've saved any of the parts that I've replaced prophylactically and I now have spare axle shafts, leaf-springs, wheel hubs, sway-bar links, etc. It's probably overkill to carry too many of those redundancies (read: too heavy) but I'm starting to learn which failures are commonplace and therefore which on-board spares make more sense to carry. The other good thing about saving the originals is that I can always switch back to the OEM set-up if I ever dislike one of my modifications.
Looking forward: I've picked up OEM transmission and engine mounts and will probably tackle that project as the weather warms in the hopes of addressing the drive-line vibration which persists intermittently, but I've got a boat in need of hull repairs and lawnmower maintenance that are both higher on the "spring cleaning" list. In July, I'm headed to the Allagash River for a five day canoe trip with some friends. It's more than 1,000 miles round-trip to northern Maine from Cape Cod and the last leg is mostly graded-gravel logging roads. I'd like to take the XJ but will need a prudent assessment of whether or not I think she's up to the task. If I break down in the north woods on my way there, I'm not sure my buddies will wait for me...