Well, this is sort of a good news/bad news/no news answer. To make a long story short, I'd get it into a shop for testing.
The reason for that is, suspicious rear-end noises can come from several sources. The good news, I think, is that it doesn't sound typical of a ring and pinion gear problem. A ring/pinion gear problem is typically more noticeable during acceleration/deceleration, and gives a sort of clanging or bucking at steady speeds, depending on how bad it is. Offhand, I would think you have a bearing going bad, but which one? That requires some first-hand investigation. And, a 1998 u-joint is not exactly new, whether it has 100 miles or 100,000 miles on it. For a u-joint, a little regular exercise is a good thing.
Proper lubricant for the differential is pretty critical. Especially if you happen to have a limited slip differential. I can't tell you the number of warranty issues/arguments/tech bulletins that have sprouted because of the disagreements over the correct lube or additive for limited slip differentials. It seems the consumer never wins in those cases.
Tires is a whole 'nuther issue. If you have a seriously mismatched set of tires, you should bite the bullet and get new ones all around. For an rv, it is a given you will need to replace your tires due to age long before you wear out the tread. Five to six years seems to be about average. Yes, tires are a major investment, but the damage done when an overage tire disintegrates at highway speed could pay for several sets of premium tires. And pay attention to tire pressure and loading, too. Can't stress that enough.
It's worth taking the time to shop around for a reliable drive-train specialist in your area. Query some companies with fleets of trucks as to whom they use and how satisfied they are. Ask about warranty on their work, and trust your instincts when you get to the shop. If your gut tells you you're being hustled, you probably are. An honest shop will make certain you fully understand everything, and won't treat you like an idiot. I used to bend a few wrenches for a living, but now I just simply prefer to pay someone else to do the work. So, if I'm treated like a moron, that's a clear sign that I know more than they do, and it's not the first shop I will have walked out of. Last dealer shop I walked out of lost not only the service on my personal truck, but service on my fleet of work trucks, and the sale of a new vehicle for my daughter. Hmmm, pretty costly mistake on the part of one jackass service writer, wasn't it? Don't let yourself be raked over the coals. There are more good shops than bad, but there are enough bad shops that you have to pay attention.
'02 Duramax CC
'02 SunnyBrook 31BWFS 5'er