Hard knock, extreme vibration at highway speeds

1999 MAZDA B4000
108,000 MILES
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CALGUARD11A
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I have a 1999 Mazda B4000 4X4. I was driving home on the freeway he other day and there was a loud knock and a violent vibration came through the truck. I got off the freeway and stopped, so did the vibration. I got back on the freeway and when I got up to speed it happened again. When I drive in 4WD Hi on the freeway or 2WD under 45 I don't seem to have problems. Any help would be appreciated
Dec 10, 2011 at 8:00 PM
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RIVERMIKERAT
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Check the U-joints and driveshaft yokes for play.
Dec 10, 2011 at 8:01 PM
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CALGUARD11A
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I'm sure I can google that. Is it safe to drive at sub-freeway speeds?
Dec 10, 2011 at 8:06 PM
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RIVERMIKERAT
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Raise the vehicle and shake the driveshafts. They shouldn't move (up and down/side to side) more than 1/8 of an inch at the yokes. Fronts are actually CV joints (constant velocity) See first image. I'm unable to find any images for the rear driveshaft.
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Dec 10, 2011 at 8:18 PM
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RIVERMIKERAT
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I wouldn't drive it too long. You can do more damage, even at sub-freeway speeds.
Dec 10, 2011 at 8:19 PM