Either way, your describing the exact same symptoms that my car had when i got supplied the wrong 6 speed and diff combination by spikey.

Car would start to vibrate a tiny bit, then more and more and more until the whole dash and the whole car was shuddering very violently.

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Get the car put on the speedworks hub dyno, thats how they found out that my car was farked ratio wise... the front wheels were turning 10% faster than the rears...

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I will get it put on a dyno to see what is going on. thanks polarbear for the tip.....if the ratio is different between the two diffs you would think it will vibrate straight away not when the car warms up and only when i do high speed. i can sit on 60-90 km all day and it will not vibrate.but once i go over 120km for 10 mins the it kick in badly..so thats why i thought it might be the diffs oil breaking up at high speed...but i will get it on a dyno.......so speedworks is the best place to take it?? will lowered wrx fit their dyno.. if not i will rise it up..(adjustable suspension)

By the way, my car could also drive to and from work.. it only happens on freeways after about 10min of driving ? correcT? Your diff ratios are out .. id say your rear diff is too low. you need a 3.9

The reason it doesnt happen doing less than 90 is that the centre diff is designed to deal with changes in torque between the front and the back and at light loads/low speeds the constant difference in power front to back doesnt heat it up enough for it to lock. Once you go on the freeway the 10% or whatever difference in speed between teh back and front wheels causes the diff to keep trying to give more power to the wheels spinning slower. This heats the diff up as if you were constantly launching and it locks and then once locked, the back wheels cause vibration as they skip over the ground because the front wheels are dragging them and they cant turn fast enough to keep up.

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Its a hub dyno so suspension is not an issue.

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Your diffs are viscous, so it DOES happen after the car is 'warmed up'.

From your description, i'd say its absolutely 100% mismatched diff ratios.

The Dynapack at Speedworks is really the proper way to diagnose the problem.

If you give us the gearbox and diff part numbers we should be able to figure it out for you without taking anything apart.

Why oh why spend extra $$$$ on dyno time when YOU SHOULD go back to whoever installed the gearbox and the diffs and get those useless buggers to fix the bloody problem!!!!!!

the rear diff is 4.111 part number 27011aa111 gearbox is sh-ty755vb1aa. do they match up??