87 4runner, completely stock, started having an issue where it shakes violently. It drives normal until about 15mph, when all of a sudden it starts shaking. Normally this is tolerable up until about 40mph, but it got much worse. Now at above 15 it is untolerably bad, especially when I am accelerating and then let off the gas. I can rev it while stopped and there is no vibration, so I don't think it is the engine. The only recent thing I had done is some exhaust work.

 

I had something like that happen to me and it ended up being severely warped front calipers. After I did a front brake job it stopped. Is it worse when your braking or accelerating?

 

could be this ^^

or tires out of balance (maybe thrown a weight), bad bearings, tie rid ends etc etc

 

Don't think its a wheel weight... at about 20 mph it shakes so hard im afraid to drive it.

 

Still could be a weight or a tire ply separating.
When was the last time you lubed and checked the suspension?

 

How long have you been using the front rotors?

 

No clue on the age of any brake or suspensions things, have had it about 6 months only and have only done oil changes.

 

Honestly I would suggest looking into getting new ones, or at least make sure its not the rotor or suspension components.

 

check lug nut torque, do you have a jack and some jack stands. jack it up safely and wiggle the tires back and forwards to see if you have any play in steering or suspension do the front and rear. then inspect your driveline and swaybars.

 

Thanks for the advice everyone; the problem was apparent as soon as I climbed under it. There are 4 bolts connecting the ujoint to the back of the tcase. 2 had falled out and the other 2 were about to fall out... a week ago I had bolts fall out of my exhaust manifold... 4 new bolts later and its good as new. I think 1 bolt fell out and then I had the initial vibrations then when the second fell out is when it got much worse.

 

Just another example of why you should do some checking on your vehicle before asking for help on the internet. there is no replacement for grabbing/shaking/banging and looking under your rig; and waiting for Chrystal ball diagnostics could lead you to continue test driving until something comes apart in a catastrophic manner.