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440 Dart 4 spd, solid motor mount on left side. 4.10 Sure grip

No noticeable vibration at low speed (town driving). It starts to be noticable at 55 (3k rpm) with the clutch pushed in (I don't think it related to the engine - I just think the solid mount makes the whole car "humm" a bit at above 3k rpm). It starts to get bad at about 65 or 70 -I haven't pushed it any further than that. Doesn't seem to get worse or better if I disengage clutch at high speed and change the engine speed from idle to 5k rev.

I'm thinking

1) Have balance checked on tires/rims - bad balance, bent rim maybe
2) Pinion/tranny angle
3) Driveshaft balance
4) Bellhousing alignment (can I check this in the car?)
5) other ideas???

Give me your ideas, obviously looking to go from least expensive/easiest to harder/expensive.

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Sounds like your on the right path. I'd check the pinion angle first because maybe with the solid mounts and the combination of the tranny mount you might be throwing it outta whack. I've seen bad tranny mounts cause that. Not wore out mounts, bad ones from the get go. A guy on here last year posted about chasing down a similiar vib like yours and after all the stuff you listed he traced it back to his brand spanking new poly tranny mount. Good luck I know from experience bad vibes suck to deal with and to find.

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check the u-joints.

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