WOW! My 94 has NEVER done that, I don't think that's remotely normal. You might hear the 4x4 engage with a single (or very short number) of clunks, but it should never sound like you're driving on a flat (which to me means the noise never stops).
I once did have a problem with the 4x4 not engaging and disengaging correctly, there are two main causes for that. Since the 4x4 hasn't been used in at least two years, you're most likely a candidate for one or both of these.
The first thing costs you no money. Pull the auto hubs. Remove the spring cams, clean all grease off of them with parts cleaner, toothbrush, etc. Clean any grease off the spindle, and reinstall the cams without grease. Thoroughly clean out all grease in the hubs, fill with ATF, let soak (at least an hour or overnight is better), drain and reinstall. Do NOT put any grease in the hub or on the spindle. Grease is the enemy of properly operating auto hubs.
If that doesn't solve the problem, take a look at your u-joints. Do a search on this forum for the proper inspection/replacement techniques. These are a very common failure, and caused my disengaging problem (which was fixed when the u-joints were replaced).
Beyond those two things, which by far are the most common issues that would cause the sound you're hearing, there are other more expensive possibilites, but definitely eliminate these two first.
BTW, I have over 263,000 miles on my 94 (put 210,000 plus on it myself), and don't get the noise you are describing.
Good luck, keep us posted.
Mike
