Noise from...the transfer case?

[Rosett]

When I bought my '92 Alltrac last year, it had a hole in the flex pipe, and when I got it fixed, I noticed a rumbling sound at speeds over about 35 mph that "seems to be" coming from the rear half of the van. The noise varies, and is not always present, but usually is at highway speeds. I've checked all the wheel bearings, the rear differential, the tires, the exhaust system. Last Friday, I took it to a shop where they put it on a lift and ran the van in gear while listening underneath. They told me that they thought that maybe the noise was coming from the transfer case. Unfortunately, the fill plug has been over tightened by some idiot, and since it was already 5 pm, they didn't want to mess with it beyond using a wrench. I plan to take it to my regular mechanic this week to get the bolt out and change the fluid in the transfer case.
The van doesn't show any of the signs of transfer case failure-no problems with tight turns, no loss of power to any wheels, etc, so I'm hoping that the fluid is just low, and filling it will help.
My plan for this van involves road trips to remote areas in Montana, and my main worry is breaking down in the middle of nowhere. From everything that I've read, it seems like, even if the transfer case fails, the van will still be driveable, at least to get somewhere where it can be repaired, so even if replacing the TC fluid doesn't get rid of the noise, I plan on using it. In the mean time, I'll be looking for an affordable replacement for the transfer case.
Advice? Observations? Thanks!

[Jonny]

Check the drive shaft joints too.
If you determine it is the TC and replacing oil does not fix it, don't put too much faith in the vehicle getting you where you want to go. You might be lucky for a while, but mine would not go up the gentlest of hills when my first TC failed.

[Rosett]

Well, I took it to my regular mechanic(I took it to the tire store mentioned above because they sold the tires that are on it, and I mostly wanted to have them check the tires), and the TC checks out fine, as do the drive shafts and U-joints. He also listened, with his stethoscope, to every moving part under the van, and came up with nothing. I guess that I'll just turn up the stereo and drive it...