great till almost 65, at 65 all He@@ breaks loose and it starts vibrating, you feel it right in the gas pedal. I changed all 4 tires, so it is deffinitly in the driveline, there are no missing weights from the driveshaft, and does not seem to be much play either. Anyway monday the shop will change the u joints anyway.
Other ideas?

 

You didnt mention anything about the front end. That is where most of the shaking problems happen. ball joints to tie rodes and bushings all can cause that. Get the front end checked.

 

the front is fine, the vibration feels more like it is deffinitly from tranny back

 

I have the same problem at 120kph its kinda like a cycle its really rough than lightens then roughs up again go faster and the shaking speeds up once you get under 115kph its not really noticable but have not taken it over 140 to see how bad it gets...
Have had whole front checked changed sets of rims up to 3 times.

They say it could be the diff or not enough rubber mounting on the engine when the 360 conversion was done..
Anyway if you fix the problem pls let me know what worked
Thanks

 

does the shaking occure under load, crusing, deceleration or just at high speeds during all three.

 

Pop the diff cover off and check it out. I had about the same problem up until mine stopped running altogether, turned out my diff was pretty much snapped in half and the gears were all chewed up. Couldnt hurt to check and replace the diff. fluid anyway.

 

load doesnt seem to change the problem while cruising the effect cycles slowly rough then smooth etc but as soon as I take my foot of the accelerator the cycle is much faster until speed is reduced to under 120kph

 

Had similar problem, Check the u joints in the shaft..

 

just a thought check you mounts

 

my u-joints went a few times, they vibrated bad downhill during decelaration, then after a while they vibrated all the time, replaced the u-joints and it ran fine. you said when you let up on the gas it get's worse, it's probably the joints because there is lower stress on the shaft when you let up on the gas and it can flop around. i also found the back joint seem to go and the front one by the tranny was good.