Intense Vibration

I have a 1996 Super Custom AWD and the entire car vibrates to hell and back when going about 60km to 110km.  If I push it past 110km it goes away completely.  I don't notice it as much on lower speeds but when I start pushing towards 60km the vibration starts to get really bad. The steering wheel is not shaking, it is only the van, combined with an extra loud humming noise that pierces your ear drums.

Things I have checked:

New Tires and Alignment - Car drives Straight
Wheels are Balanced
Replaced Transmission mount to Chassis
New Front Shocks (waiting on the rears)

When I replaced the transmission mount the vibration became slightly less, but its pretty bad and I feel unsafe to drive at higher speeds right now.

I did check the rear u-joints and driveshaft and there doesn't seem to be any play.

One person I talked to recommended that I needed to balance my driveshaft which I am going to work on doing ASAP.  

Any other suggestions would be great before I have to take it into a shop.  Would love to have the parts beforehand so van isn't just sitting.

Thank you!!

Engine and gearbox mounts possibly.

There’s an app called VibSensor I have on my phone that uses the phone’s 3-axis accelerometer to track and graphs high frequency vibrations. It’ll give you x, y, z axis vibration frequencies over a period of time. You just lay your phone on the seat, dash or console, hit Record and it shows in real time like a 3 way seismograph. With the graph and data you’ll see the larger freq vibration very clearly.  It’ll enable you to match the Hz frequency to either your driveshaft or wheel rotaional speed based on engine revs or wheel rotations. If you don’t know gear and diff ratios just do sums based on wheel circumference at speed travelled. That’s the fast, easy way. You don’t need to be exact, the driveshaft spins approx 4x faster than the wheels so the frequency is distinctly different.
The point is to isolate whether the vibration is from something before or after the diff. Engine/trans/driveshaft or wheels/axles.

Just reread your post and a loud ear-piercing hum indicates it could be bearings somewhere - Transmission; center-diff; front diff; rear diff; wheel bearings.
My money (but only a dollar!) would rear wheel bearings, center or rear diff.
Rear wheel bearings are an absolute b*tch on these (any Toyota like this) so often get deferred. Mine also didn’t seem worn or give noises typical of wheel-bearings yet changing them removed vibration and what sounded exactly like a rear diff whine.

The vibe sensor will allow you to isolate it somewhat.

Piercing noise would indicate that your troubles are related to something that rotates...

I would suggest to check rear wheel bearings. It is easy to diagnose with putting your foot on rear wheel and getting the car to rock sideways in a certain motion. When the motion is right, you will feel the play in bearings (if they are faulty). Other tell tell sign is soiled brake drums from rear dif oil leaking though rear bearings/seals. If you can see oil seeping through rear wheel bearing hub/brake drum meeting point... process has started. If oil got through seals, you as well change the bearings as the work involved in changing seals is pretty much the same as changing bearings.

While you at it, you might want to check rear/front differential oil. Is it up to level, is it clean, does it have any metal particles in it?

Well, check the oil level/quality in transfer case and transmission as well.

You can check front wheel bearings same way... but do keep in mind that front wheel bearings are adjustable, so you might want to look for differences in how they feel when you rock the wheels back and forth.

Another issue (probably not this time), is the orientation of the spiders in the driveshafts must match each end, or the shaft rotates unevenly when set at an angle.

Good point GPW. If either front or rear driveshaft have been out it’s a possibility the shafts were separated and reinstalled without attention to the alignment marks. They can also appear to be in-line but they have an odd number of splines on the sliding joint so if the ends can be one spline out of alignment if flipped 180deg.

Related to driveshaft play - the input/pinion bearings on any of the diffs could be shagged-out, or the nut loose. (The nut that holds the diff shaft and flange/spider on.)

Thank you guys for the comments!

I went ahead and pulled the front and rear driveshafts and found the u joints on the fronts to be shot. Dropped them off at a drive shaft shop and got everything replaced and both shafts balanced for $100.

Now van drives like new!

Nice work. It just reminded me that when I first bought my van it had a horrendous shake at 50-80kph abd random clunks on bumps. Turned out the front right driveshaft outer CV was knackered but the vibration was the driveshaft bearing where it goes through the chassis. I’d forgotten about that - it was 15 years ago.