Drive train Vibes

[Micah72]

So I've had this drive train vibration that has been getting worse as the weeks go by.
It starts around 30 mph and continues to about 45 and then picks up again around 65. I was thinking maybe a drive shaft is out of whack but what confuses me on that idea is when I am up to speed, I can push in the clutch and it goes away. I would think if it's a drive shaft it would continue whether there is torque on the drive train or not.
Anyone have thoughts on where I should start looking?
It's a '90 GMC jimmy, 350, SM465, 241 T case and D44 front with 12 bolt rear.

 

[mechted]

Unless you just recently changed the driveshaft angles, or bashed a shaft on a rock, its probably not that.
Check your u-joints. Fairly likely that one (or more) is dead.

 

[dyeager535]

What kind of shape are your motor/trans mounts in? Easy check.

 

[Micah72]

The whole engaging the clutch and vibration going away thing was confusing me but as I was doing more research on it last night and found that most people with the problem have either bad mounts or U joints. so i'm going to check all that out tonight.
Thanks!

 

[dyeager535]

The whole engaging the clutch and vibration going away thing was confusing me but as I was doing more research on it last night and found that most people with the problem have either bad mounts or U joints. so i'm going to check all that out tonight.
Thanks!

If you find out you have bad mounts, IMO go with poly. Unless they are GM pieces, I have not found aftermarket rubber mounts to hold up well at all, and I'm not alone in that experience.

Preferable to do both motor mounts and trans at the same time if you go with poly. I took my poly motor mounts out when I replaced the engine a few months back, probably 5-6 years old, nothing to indicate any wear.

 

[Fordum]

Probably U-Joints. When the bearings go bad, they don't rotate well under load. Also, if the bearings are worn enough, it lets the cross move sideways in the caps enough so they are not lined up like they should be.
When the joints are under load, such as when accelerating or decelerating, those problems are aggravated.
When you push in the clutch and coast, all the load is off the bearings, and they float into alignment and the bearings don't have any load and so rotate freely.

Bushings can cause similar symptoms, but don't go bad as often, and when they do, don't usually get worse as fast unless they are completely disintegrating.