My truck starts to shake violently anywhere above 70 mph. I 'm looking for some ideas as to what it may be. Oh and its a 98 4dr 4wd blazer if that helps at all.

 

Alignment, wheel balance, I recently had to replace a wheel hub on my blazer.

 

bent rim or really f'ed up tires too

 

wheel bearing, cv joint or weights, tir rod end, ball joint, probably not weights because you'd notice it at low speeds, like 20 - 40 mph & 70mph? damn man, ease up a bit lol

can you tell if it's front or rear end? might be a counter balance weight that came off the rear drive shaft or if you've been in the mud recently something on the inside of a rim (it would be small to be noticed at higher speeds)

try the NS/EW push/pull when the front end is in the air (lifted by the frame, not suspension), too much movement will tell a lot

 

Dear Blazer owners,
The above mentioned thread "vibration 60-70mph" has your answer.

Sorry to say, on Page 2 of the thread, it is reavealed that CHEVY actually knows of the 60-70 mph vibration and is not willing to do anything about it. They seem to have built it into the car.

The original poster of the thread vibration 60-70mph recently posted that he fixed the problem with new REAR AXELS and REAR BEARINGS.

I have suffered with the "bad tires" or unbalanced tires excuse for 4 years.
The above thread is a god send.

Moderator might consider consolidating all high speed vibration threads into one thread with comments from the moderators that this can't be fixed.
I personally find it curious that the rear axels being bent could cause such a common problem. ..

God bless