Well, I'm getting around to diagnosing this. It starts at around 55mph and goes away over 70mph.

I pulled the front shaft off yesterday (it's been replaced with a serviceable one) and there is zero play in any of the 3 u joints, so I don't think it's in the front end.

There is no play in the rear driveshaft (looks to be original).

Does this leave me to the diff? I've not pulled the rear shaft or jacked up the truck to see about play, but that's about all I can figure.

Ideas?

 

Curious - Have you checked the rear tires to see if they are still balanced or if there is any play with the wheels (i.e. bearings)?

 

You have to lift the truck and make sure its firm. The drive shaft won't budge even if the joints are loose when I tried it when wheels are on the ground. But, when raised, you can see the where the clunk originates once you rotate them. Like what Friday Night said, also balancing is a factor. Also some factors that can trigger vibration if the brakes are warped.

 

Tire balance is perfect. They were literally brand new when I picked it up and I've done about 4K miles with them and show zero wear. Steering and alignment are also sorted according to GBR when I had them give it a once over after I got home.

Jacking it up and doing the tests are next steps. It still has the rotoflex or whatever in the back end and a non serviceable unjointed up front.

 

I will bet you a $1.00 that the rotoflex is your problem. I've had this happen many times with RRC('95), DIs and DIIs. Most of the time the cheap aftermarket ( Britpart, Allmakes, etc) roto's fail fast, but old OE GKN joints will also cause vibration problems. And the cracks in the rubber do not have to be major. If you get the roto off the truck and twist it you will see tons of small stress cracks.

So 2 options; Convert the rear drive shaft to a (2) U-joint shaft, or get a NEW GKN Rotoflex. If you choose the rotoflex... Trust me, you ONLY want a NEW GKN (#TVF100010G Made in Germany). Good Luck!

 

Wel called it the "guibo" in my E30 days, but very good info. Thanks!

 

Initially I would go for rear wheel balance or an out of synch/balance driveshaft, but, the Watts linkage can have a strange deleterious effect similar to a vibration so check all your connections on the Watts linkage (5#) and whilst under the truck checkout the shock bushes even if you have air suspension. Watts linkages play games with vibes and stability, it sounds crazy but it's happened to me.

 

The GKN Rotoflex is good to go, it's the best.

 

Haha. Yep. Same in my e39 days!

 

Lucky8 has the rear conversion kit if you wanna get rid of the rotoflex but if not go factory and not cheap on it. It will bite you in the *** if you do.