At about 55mph my jimmy hums really bad. It starts at about 45, and completely stops at about 59mph. The humming sounds like really bad super swamper tire noise (I've ridden in a ragtop 35in superswamped jeep before- that bad). This is in 2wd. When I switch to 4wd the noise totally disappears, but a very very slight squeak.

What could this be? I think I prefer the slight squeak to the hum.

I would check your hub bearings first off.

http://www.automotiveforums.com/vbulletin/showthread.php?t=321920

Several of us having the same problem. I only notice it in colder tempartures after extended highway driving. The dealer thinks the vacuum switch might be freezing up or the actuator cable binding.

I have been having this problem for about a year now, winter and summer. The problem is primarily in 2wd, though it could just be that 4wd isn't properly disengaging, but I don't think so. I can tell when it's engaged because my steering binds up a bit when turning into parking spaces.

I don't know much about 4wd bearings so I will look into that.

Check your transfer case fluid level and maybe flush and refill with new fluid.

I hate getting off task, and I dunno the truth in this, but in the astrovan forum on this site a number of people said they add this "blue additive" to the transfer cases of their awd vans because they lockup real bad when turning. I know it because my parents 2000 safari is horrible- You can hardly turn a corner without the wheels locking and the thing jerking violently. It's supposed to work magic because it helps "lubricate the clutch pads." I'm no genious wit 4wd and awd, but would this stuff help our 4wd transfer cases?

I just had my transfer case drained and filled (they overfilled it but I emptied it out to the right level). I don't think this is the problem.

I had same problems for a while took a while to figure it out
untill one day crusin along about 35 or so when all of the sudden my front axel locked in all by it's self yikes what a noise! funny a freind mentioned the vacum actuator may be going bad well he was correct

How much are one of those vacuum actuators? It seems like a common fix that everyone ends up doing on these things. Should I go to a dealer for this part?