diagnosing a vibration

[ninchi]

2001 TD5 110 wagon, 300000km. Just started noticing a vibration which:

- isn't noticeable until over ~60km/h road speed, and seems to increase in frequency with road speed
- seems to be a fore-back or up-down vibration, rather than left-right shaking
- is not related to engine speed
- occurs in both 3rd and 4th gear
- is worse when under power, over about 10psi boost. barely noticeable when coasting at low boost

Potentially ruling a few things out:
- I thought it might be a wheel balance weight fallen off, but that would be independent of power/acceleration
- I checked the wheel nuts, all torqued to 110 (steel wolfs)

So that leaves me with:
- bearings
- diffs
- propshafts incl a loose bolt
- transfer case/gearbox output shaft

Any thoughts? tia

[JDNSW]

Most likely is propshafts (worn universals, loose or missing bolts or worn spline). Vibration from these will be around three and a half times faster than anything related to roadwheels.

But the other thing to think about is something that is vibrating sympathetically with residual prop shaft vibration. This sort of thing will usually be most prominent at a particular speed.

There are other possibilities, but all much less likely. If lifted it could simply be a matter of excessive U-joint angles, perhaps exacerbated by incorrect alignment of suspension.