3-second answer: The driveshaft's slip yoke (where the shaft slides into the trans/TC) clunks on takeoff when the splines are dry or the yoke's been run dry and worn. Fix: unbolt the shaft, grease the splines (the right grease — moly or the Ford spline grease), reinstall. If the splines are visibly worn, replace the yoke (~$50).
Why it clunks
- The slip yoke slides in/out as the suspension moves. Dry splines stick, then release with a CLUNK on throttle or gear engagement.
- A worn yoke (grooves in the splines) clunks no matter how much grease — replace it.
- A bad pinion/u-joint can feel identical — check u-joints first.
The service
- Driveshaft out (mark orientation, support it).
- Pull the yoke out of the trans tailshaft (or TC on 4x4) — catch the fluid (it'll drip a bit; fine).
- Clean the splines, pack with spline grease (or the Ford yellow grease — NOT wheel bearing grease).
- Reinstall, torque the bolts, grease the u-joints too while you're at it.
- Test: if the clunk's gone, done. If it persists, the yoke's worn — replace it.
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Threads
- FTE search: slip yoke · driveshaft clunk