Drive line clunk is caused by the male and female splines of the drive shaft transmission yoke "catching" on each other. The drive shaft has a yoke on the end into which is pressed one half of the universal joint. That yoke has a female splined member which slips over the male splined output shaft of the transmission. Normally these two splined members move with respect to each other when you go over a dip in the road or otherwise cause the rear end of the vehicle to go up or down. Since the transmission is mounted fast to the frame member of the car and the rear end floats up and down with respect to the frame, the drive shaft must become longer and shorter to compensate for this movement.

When the splines are new and well lubricated they move easily over each other, the female member moving in and out on the transmission spline. Eventually they dry out and the edges of the female spline become sharp and bite into the male splined output shaft. When this happens, and when the drive shaft is trying to re-enter the transmission, the splines lock together temporarily for an instant. Then the splines release their hold on each other and the "clunk" is felt throughout the chassis of the vehicle.

To eliminate the clunk you can file the sharp edges of the female splined member so that they no longer can catch on the male spline. You can also put a small amount of lubricant on the splines however it must be compatible with whatever type of transmission/fluid your vehicle uses. There are some silicone based greases that will do just fine.

Ford issued a seal kit a long time ago which was inserted into the female member and which prevented the lube from getting out of the region and co-mingling with the transmission fluid. I don't know if any other manufacturers have such a kit but it would be worth asking at the parts counter.

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