Well since I bought my 77 F150 SWB 4x4 (351M, married NP435/NP205) in January, i have returned the truck to a very reliable driver. Probably have 2-3k miles on it, without a hiccup. One thing I have noticed since I got the truck, it was very noisy when you applied power to the drive train. A very heavy metallic clunk that shakes the entire truck. Would also get a vibration in the drive line when coasting at a very specific speed(around 48 MPH). I attributed it to worn rear shaft U joints and likely mushy body mount bushings allowing it to reverberate theough the truck. Replaced the U joints and centering yoke, amd surprising they didnt seem too bad. The centering yoke was a little suspect, but the u joints were pretty tight. No wild slop that would make the kind of noise
im hearing. Well truck still makes the clunk after the joints, although the vibration is improved, but still there. The rear output seal of the NP205 was leaking so i replaced it too. Im beginning to think the transmission mount is seperated, allowing for the loud clunk when i unload the clutch to apply power. If feather the clutch/rpms very carefully in between gears, i can eliminate the noise. I think the motor
mounts are OK, as I can see the motor with the hood up from the drivers seat, and there doesnt appear to be any abnormal
moving from the engine when its revved or put into forward or reverse. Dont have a camera to watch the transmission unfortunately. Any comments or ideas? Plan to check this evening or tomorrow if i can get it on a lift.

 

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Should be easy to get a close idea with the manual. If the trans or tcase mounts are broken the shifters should move dramatically in conjunction with the sound.

 

Well most of the time when I hear/feel the clunk, the shifter is in my hand, and I don't feel it wanting to move or feel the clunk in the shifter, which may debunk my mount theory. When I did the U-joints, the pinion bearing didn't have any slop that I could feel, but I did notice there was rotational play in the yoke before the rear axle engaged, 1/8th turn maybe. Ring and pinion gear worn excessively?

 

Can’t get the truck in a lift, but was able to look at it today. With the truck off and in gear(parked headed slightly up hill) I can rotate the rear tire forward and have a lag in the pinion movement(starts stops starts again) which I assume to be wear in the ring and pinion. The slip yoke that bolts onto the rear end, of which the splined portion of the driveshaft also appears to be worn and allowing the angle between the slip yoke and drive shaft to change minutely. I gather than I need a new slip yoke and/or spline.

 

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