I'd bet a shiny nickel that it's one (or both) of your rear half-axles. Driving long distances at high speeds with a car full of heavy luggage really puts a strain on these U-joints.
I had the exact same problem on the way home from my driving trip last year. I was on the way hom from D.C. and had made it as far as Georgia when the problem developed. Thump, thump, thump, thump... pull over to the side of the road, everything looks okay, get back on the road and it's fine for about 5 miles, then thump thump thump again.
I finally pulled off at an exit and went into a gas station parking lot.. and the right half-shaft was SMOKING AND DRIPPING FLUID! ack! I managed to limp back home at 45mph, making a 14-hour trip out of what should have been an 8-hour trip. By that time, the rubber boot in the middle of the half-shaft was completely disintegrated, leaving the joint exposed.
A new half-shaft will probably run you a few hundred. Installation isn't so bad, especially if you have the service manual. i think there's another thread in this very forum with brief instructions on how to do it. I found a used half-shaft for $120.
'83 240D, 250,000 miles