I have a 1998 4.3 Blazer. 2 Door 4x3. I have a very hard vibration when I get it to about 65-70. But I can only feel when I have my foot on the throttle or have the cruise control on. The second I let off the gas, it stops. I have check the tire balance and alignment and that is not it.

 

Hmm, when I reach high 70's and beyond my blazer starts to shake which I would consider normal for any "older" vehicle I guess. When I ride in my firends 98 chevy 1500 at 70 mph the thing shakes also. But its weird how yours only shakes when you press on the gas. I would say its a tranny issue...output shaft?

 

Where do you feel the vibration, in the floor, seat or steering wheel? Does it go away if you go 75?

 

Sounds like the start of universal joint problems. I'd give a thorough check of all of the joints on the rear driveshaft.

 

This is not the same problem as yours, because it would not go away when you let your foot off the gas - however just a quick tip: Two days ago we got an ice storm and the ice was heavy enough on one side of the wheels to create a vibration at highway speed. You can get the same thing from snow build-up. The next day, the ice had melted and everything back to normal. Just gets you worried for awhile.

I don't think there should be vibration at highway speeds just because it's an older vehicle. I've also had it from a broken belt in the tire, which won't show in a static balance but will in a dynamic balance. It takes very little weight shift to create a problem.

Doug