How many miles are on the touring and what has been changed over the years?

As Bret said, it could be the u-joints. It could also be a lot of worn out or broken parts. Some other places to think about is the subframe bushings, axle bushings, the brake rotors, if the front wheels are not vibrating, then swap them to the back (I have seen 2 different sets of wheels unbalanced as they were both used for the same axle), CV joints, gunk in the inner brake drum assembly (parking brake), Guibo/flex disc, bad output shaft bearing on transmission, bad flange bearing on the differential, control arm bushings, sometimes bad rear strut mount, bad transmissions mounts, and perhaps a few more.

Now a few questions:
1) Is there any delay or clunk when you accelerate at low speeds or from stop?
2) Do you feel the vibration more from the seat or the cargo area?
3) Do you feel any vibration in the steering wheel?
4) Can you describe in more detail the noise, clunk, when it occurs, any other times, when you began noticing it, how loud, pattern, etc.?

Here is a link from doing a search for some ideas and a procedure to check your CV joints: http://www.bimmer.info/forum/showthr...=3061#poststop

Good Luck.