Noticed that my '90 Trooper makes a sort of whirring/ grinding sound (I know you cant hear it from there...) when I cruise down the easy parts of the trails and hit about 30-35 mph. It is not affected by rpms, only speed. The hubs and cv joints I believe are not the culprit, as I've just serviced them and the noise remains. (I am in the process of changing the clutch because it is time- 155,000 miles and this is the first change as evidenced by the isuzu logo stamped on the clutch disk and pressure plate, not to mention that its worn down past the rivets and the pressure fingers are eroded at the tips) I thought that the noise may be coming from the driveshaft. I called a cv joint and axle specialist and asked if they balance driveshafts. He said no, and asked what was the problem. When I told him, he said that I should never go faster than 10-15 mph in 4wd and that the driveshaft is not the culprit. Is this true? Is 30 mph in 4wd too fast for this set up? My old 90 cherokee would do 50+ mph off road without a hiccup. Any advice would help- thanks.

30 is about as fast as you can practically go in low range, due to gearing (20 if you have 3.07 t-case gears <img src="/forums/images/graemlins/smile.gif" alt="" /> )...in high range 4WD I have gone highway speeds with no trouble (slippery conditions) many times, many different vehicles.

What does your owner's manual say?

If you can go 40+ mph, you don't need to be in 4wd and are just causing unnecessary wear and tear on the drivetrain. Check the fluid level in your t-case. The needle bearings on the input shaft will start talking to you if the fluid level is too low; and they're not fun to change.

I second the fluid check. Mine made some funny noise a few years ago. Dang thing was dry as a bone!! <img src="/forums/images/graemlins/scared.gif" alt="" /> Filled it up, noise went away.

The t-case fluid pours out when I remove the fill plug, so that's okay... However, I changed the u-joints (which looked okay inside and out) in the front driveshaft while it was out for a clutch change and the noise has gone away. Now all I hear is the comforting low-pitched humming of the front diff gears.