When it's happening, you can see the hood shaking - most noticeable directly over the front wheels. It does seem to be worse on rougher road surfaces, but I'll have to check that more carefully this week to be sure.

 

Well you gotta believe it's front shocks. OTOH, my slightly less than perfect front tires do the same thing. I have 9.5" wheels (with 275's) on the fronts. That's a LOT of meat to get perfectly balanced. In my case, I think part of the issue was how long the car sat w/o running. I don't find it a problem unless I go over 70mph.

I have new front shocks too. Rebalancing the fronts removed about 80% of the vibration but some is still there. You could try pumping your tires up to max pressure + 2-3psi. Getting more of the tire off the ground might help you isolate whether the tires are an issue (even after balancing). Anything short of new tires probably won't prove diddly-squat.

 

It could be as simple as air pressure in tires. below 35 psi my 95 has a vibration in it.

 

At the last shop I worked at,I balanced my tires on my vette with our "regular" balancer & still had a vibration/shaking at highway speeds.I brought it to a shop that had a road force balancer & that made all the difference.Did the shop check for a wobble or hoping while the tire was being balanced?Could be a defective tire.Did this problem occur after any work was done or parts were replaced?Did you throughly check your suspension?I am working on a car tommorrow that shakes while driving due to bad/worn out inner tie rods

 

Who has those? Cost? Sounds like a good option. With mine, it just seems like I'd need a balancer that spins faster/higher. NTB (where I go) says they get the occasional person wanting the same thing. They couldn't understand why.

I guess they never drive above 70 or go to the track.

 

With a LSD, 180 is not an issue.

What I found out was the u-joints will "checkout" just fine until removed and inspected. If you look closely around the grease seal you may find the slightest signs of "rust" coming from under the seal.

This is indicative of a failed/ceased needle bearing pack. It will produce a small yet annoying vibration.

The batwing rubbers are seldom the cause unless completely disintegrated.

 

Road force balancing:
Measures vibrations by placing a tensioned spinning meausrement wheel in contact with the tire. It can detect vibrations caused by run out (out-of-round) conditions, as well as normal weight distribution problems. Takes the guess work equation away from a tire technician. Do a google search for a list of local shops that use these types of machines.

Road Force Video:

 

Interesting stuff guys. I'll see what I can do, but things will be on hold for a month or so. I suddenly have medical stuff to deal with concerning my wifes dialisys.

 

If you exhaust all those other ideas without a solution, take a look at the front brakes. Caliper sluggish, warped rotors....

 

I balanced my tires on my vette.& got the readings to 0 on both sides of the tire.I still had a slight vibration at 70mph or over.I brought it to a place that is known for their equipment.They showed me that the tire was balanced the traditional way but with road force appied,there was 10 lbs rotating on end of tireThey had to break the seal at the tire & rotate the tire to a different spot on the rim to get both the "perfect"
spots on the rim & tire to balance/cancel each other out.It took a few tries but what a differenceI believe it cost $160 total($40 a tire) but was definately worth it.That machine will also tell you if the tire is bad,rim is bent,etc,etc.Check it out & I recommend it if you drive your car at high speed like.....

 

Check horizontal & vertical movement @ the drive shaft yoke. The tailshaft bushing may be worn allowing the front u joint to rotate in an eliptical orbit. This vibration can normally be felt in the floor pans, seat etc. Severe cases cause body shudder.

Intermittent vibrations are normally caused by a tire(s) & can be heat related.

IMHO: The belts are probably moving around in the tires.

As suggested, road force balancing IS BEST & may identify a problematic tire.

Before spending $$ on balancing insure that the wheels are straight & true.

The following check is time consuming & tedious however it may save some tail chasing & identify the problem area.

Using a a dial indicator check each wheel for lateral & radial run out. Check the outside & inside of each wheel @ the wheel lips. IE: The horizontal & vertical flats next to the rolled edge. Before indicating, remove the wheel & clean any corrosion from the face of the hub & the wheel mounting surface.
To index a wheel: Mark the wheel & one corresponding wheel stud, indicate the wheel & record the readings. Move the wheel one set of bolt holes & indicate it again. Continue around the bolt circle. Install the wheel at the position on the hub that had the lowest amount of run out. Mark the wheel & stud & always reinstall the wheel in that location.

IIRC, the 89 shop manual allows .040 run out in either direction, lateral & radial.

Luck with your hunt.

BTW, enjoy reading your posts mcm95403, you have brought some enthusiasm to the forum.