yeah boof i think i might try that...

fingers crossed that it solves the drama...

hopefully i can get it all sorted... would love to bring it along to the GOR cruise

thanks for the tips so far...
ill keep you's posted..

ANGE,
I had the exact problem with my rally coupe, it took me a number of months to sort it out because such a vibration is hard to diagnose.
After much mucking around it turned out to be the preload adjustment on the pinion bearing. This is what gives you the vibration when you unload the driveline and it is in between drive loads and throttle lift off.
Hopes this info helps more than one member.

Hey ANGE (of topic) what does ur 12a do down the 1/4 mile?
Is your tail shaft not bent?

I reckon Phunk's right on the money with the pinion angle...... It's critical that when you set up a pair of variable angle couplings (in this case your uni joints) that the angle between the drive shaft & input end (gearbox) & output end (tail shaft) are the same.... It's to do with 2 different rotating inertia values & stuffs up your uni joints REAL quick if you don't get it right..... As per your vibration!
You could maybe sit something in the back of your ute to simulate the rear springs under load & then measure your gearbox to driveshaft & driveshaft to diff angle. If they don't match up you may have to cut & turn your diff mounts........ The shims might work though

i really hope you guys are right with the angle theory...

but let me clarify..

my engine and gearbox are on a slight downhill from engine to gearbox...

this means my output shaft is slightly pointing to the ground..

if i angle my diff pinion down doesnt that mean that my two angle's would be opposite?? not the same?

hmmmm .... yep and hopefully when you are on the gas it will lift the pinion gear to be pointing up more towards the box output shaft ..... but you said it was under more of a "cruise" situation yeah ?? in that case i'd give it a go with the washers at the front to and see if it changes the vibration gettin confused now ... but if the tailshaft bloke reckons to lift the box then droping the pinion should achieve the same ??

i did try putting 2mm spacers at the rear to point the diff downwards...

it had no noticeble result...

putting bigger spacers would make the diff pinion angle start to look ridiculous

.........?????? maybe give it a go with the shims at the front ??? or tailshaft balance again ?? there used to be a bloke (real good) had a couple done by him years ago, but he's the other side of the bay - Seaford, if he's even still around maybe even do a ring around to a couple other tailshaft places just to give em a quick quiz in regards to the angles of dif and box alignment ??

yeah gotta get the tailshaft looked at...

but if the shaft was out of balance wouldnt it vibrate constantly at speed? even when under load.. because my ute drives as smooth as at 120kph with the slightest bit of accelleration..

im concerned with the play in my output shaft... anyone know the tolerences before vibration becomes a factor?

Ok, from what you had previously said, I was under the impression the box pointed upwards towards the back.
In the case that your box points down towards the back, your pinion angle needs to point up towards the front. Both box and pinion need to be on the same angle. Its the same deal with side to side angle - since the diff pinion is straight forwards, the engine/gearbox has to point straight back. ( Pushing the gearbox output shaft to the side without moving the engine to the side with it, will also result in driveshaft vibration.

Most hardware shops will sell angle finders ( like a vertical compass that measures in degrees )
Just using the eye isnt acurrate enough.