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?

 

I'd check the U-joints on the driveshaft and the driveshaft itself. Maybe a balance weight came off after 30 years.

Since you don't feel it through the steering and you checked the rims and tires it doesn't sound like it would be suspension related. Maybe check the rotors, but I think you'd feel that through either the steering or the brake pedal.

 

It sounds like a wheel balance issue. Does the place that balanced your tires have a road force balance machine?

 

Most jobber shops don't pay attention to rim or tire runout.
Put the rear end up on jack stands have someone sit in the car and run it up to 30ish MPH while you observe the rims and tires for hop and wobble.

Remove the rear wheels and look for crust or debris caught between the rim and hub interface. A little chunk of rust will cause wobble.

A hung-up caliper piston or slider pin or drum brake shoe or cylinder will cause this too. A collapsed rubber brake line will cause brake drag which causes high-speed chatter.

Grab the drive shaft at the rear and wiggle up/down...side to side. It should not move. The front of the driveshaft will move on the yoke splines the u joints should not move.

 

oldcutlass, I do not know the answer to that question. I've been stuck on nights for a while but I will get back to it a week or so. The shop who did my tranny is going to take another look at the U-joints and driveshaft balance after Memorial Day.