I drive back and forth to New Orleans several times a year, which is about 375 miles each way. (6 hour drive each way) and I ALWAYS bring tools. I carry a bag with pretty much every socket from 7mm up to 22mm, a few ratchets, a breaker bar, and a cordless impact gun. Oh, and zip ties, ALWAYS carry zip ties and some snips to cut them with. One time during a road trip in the wife's Altima we stopped in Baton Rouge for lunch and heard a scraping/dragging sound under the car. It was the damn plastics lower engine cover or splash shield dragging because the cheap plastic pop rivet things came out. I got dirty laying under the car, but I saved the day by securing it up to the body with zip-ties!
Another thing, you mentioned that you serviced the transmission and rear differential yourself. Before the trip it would be a good idea to go over all that one more time and double check that ALL bolts are still torqued tight. If you have a white or yellow paint marker, draw a line over each bolt head so that you can just visually check it in the future to see if any bolts worked their way loose. If the paint line is broken, then you know the bolt backed out a bit. I ALWAYS check this prior to a long drive because a few years ago I had a brake caliper bolt come loose on me during a drive home from New Orleans. The rear caliper bracket bolt popped out at 75mph, then the caliper tilted down since it now only had one bolt, and it began rubbing on the inner wheel barrel. I head the "ping" of the bolt popping out, then the sound of metal rubbing on metal. I instantly became alarmed and tried to pull over and investigate. As soon as I hit the brake pedal, the caliper ended up digging into the wheel even harder, and instantly cut the wheel open just like a can opener cutting a can open. I still had no clue what the **** happened at this point, I thought I had a blowout. But then I lost all brakes! My pedal went ALL the way to the floor. Luckily I didn't panic and I was able to safely slow the truck down and guide it to the shoulder. After looking at the cut wheel, busted brake line, busted rear shock, then seeing a caliper bracket bolt GONE, I figured out what happened and the noise I heard made sense now. A good Samaritan stopped to help as he thought he was going to just help me change a tire. I actually had all the tools to fix it, but I didn't have the PARTS. Thanks for smartphones, I was able to track down some parts stores somewhat close to where we were, and called to see if they had a brake caliper bolt and a brake hose. This gentleman who stopped ended up driving me about 30 miles each way to get these parts and some brake fluid and stuff. Then I patched up my truck and put the spare on and limped it the rest of the drive home on a busted shock. Ever since then, I ALWAYS carry tools, and I draw lines on bolts after I torque them down, so I can visually check and see if they have started to come loose.