1990 Olds Custom Cruiser shaking from 55 on up.

I have a 1990 Custom Cruiser and it has been shaking starting around 55 on up and worse with speed. I feel it mostly through the seat, not much in the steering wheel. I've had the wheels rebalanced and the tires inspected and nothing changed. I replaced the rear shocks with air shocks but that didn't change anything. I haven't replaced the front shocks yet but they don't seem bad. I couldn't find any steering play or loose linkage. Could the U-joints or driveshaft cause the car to shake? They don't clunk when shifting. What else should I be looking at on this style Custom Cruiser?

 

Did you inspect the tires for excessive wear on one side of each front tire? My '89 CP had a bad tie rod, and looking at the tire showed me to look at both at tie rods as well as ball joints and control arm bushings.

 

No, the tire place that installed them inspected the tires but I'll take a look.

 

The reason why I brought up the tires was the ball joints. I've encountered some customer cars who had bad ball joints, but another shop missed that.

 

I did do the wobble check while it was jacked up for the oil a week ago and it had no excessive play up, down, or side to side on either side.

 

That's the problem. The "wobble" check is only for full-out worn ones. Marginal ones require a different method. You jack up the corner on the lower control arm, then use a big prybar and lift the wheel at the bottom position to see if the lower ball joint has any slop. Jacking on the LCA 'unloads' the lower ball joint, making it possible to check for slop in it. The upper ball joint, you simply press down with the prybar between a fixed point on the frame rail and the top of the upper control arm's ball joint. If it presses down, it has wear. You can also open the hood, lean on the fender, and bounce that corner, while a second person looks at the wheel; if there's a fair amount of wear, the steering knuckle and ball joint will deflect on the push down.

 

Did not know that method.

 

I got it from the 1970 Olds service manual.

 

I had a warped rim that would be fine until I reached about 63MPH then I would get feedback through the steering wheel and the rest of the truck.
Your problem to me sounds more like the U-joints or driveshaft if you are feeling it in your seat? I failed to center my u-joints once when working on my truck and when I reinstalled the driveshaft it wobbled but it was so far off it was apparent right away. Getting the driveshaft balance checked would be a quick affair.