So, I noticed this today in my 4kq. I will start out in 1st gear and then shift to 2nd, then the car will shudder until a certain speed. Once I hit 3rd, 4th, and 5th gear, its gone. I tried to redline 1st gear and it started to appear around 4500rpm, then not so much in 2nd. So it seems to be speed dependent.
Kinda weird, driveshaft bearing?

Drove the car to a friends house tonight. When I entered his addition, I was in 2nd, so I let the car slow down, then I gunned it, the judder was REALLY bad. I also hear that metallic noise I heard a while back, it never went away, it was sporatic, but now it happens all the time. The judder is definately towards the rear of the car.
Im gonna put the car up tomorrow and see if I cant find anything else binding or catching.
Any help?

but if it's coming from the rear... i have no idea, i have no experience w/ quattro

The possibilities are you have a stuck diff lock (you can move it manually from under the car) or if its more of a thumping noise it could be a rear differential mount - when mine went I had a thumping noise. Usually center bearings whine on their way out but, I've seen them on BMWs go to the point the drive shaft hits on the tunnel. Time to get under the car and start pushing around to see if there is anything obvious. Good luck...

I tried locking the rear diff, but it still happened. Two of the 3 rear diff mounts are new, I just havnt gotten around to tackling the rearmost mount yet.
I was lazy today and didnt even go out side, so I'll check next weekend.