I have the original 8-1/4" 3.55 on my 1980 D-150 which has 244,000 miles on it. Just recently it seems to slip 1/4 to 1/2 a rotation as I ease of the clutch going forward. It feels like the wheel spun, but I don't think so. It doesn't do it everytime. Is this a symptom of a rearend that is worn out? Is there something I can do for it? Yes, it's got fluid.

bwhitejr

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'72 Duster (Performance 360)
'83 Ramcharger (Performance 318)
'80 TrailDuster (360)
'80 D-150 Truck (See Below)
CompCams 252S, Holley 390cfm, Offy manifold
Ported, Polished and Gasket Matched
P4286813 Springs,0.040 Overbore,
0.090 Shaved Head

I would first check U-joints and make sure they havent worn excessively, And the way to check is park vehicle on LEVEL surface, block wheels to keep vehicle from running over you if there is a chance of it rolling, put vehicle in neutral with parking brake set, and try to turn driveshaft by hand. Look for play at all the ujoints for slop, in carrier bearing, (if so equipped) and then focus on rear end... If u-joints and carrier has no play and the driveshaft is actually turning the pinion of the rear-end, then its time for new ring and pinion gear....

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MRO....

Cheap, Fast, Reliable.... Pick 2...

Be sure to check your engine/trans mounts as well. Bad mounts will let the powertrain move around- most noticeable when you just engage the clutch & start to roll forward.

You might need to unbolt the driveshaft from the rear axle to get a good "feel" for U-joint condition- just don't drop the 2 (loose) joint bearing caps when you pop it out of the axle yolk- & don't forget to tighten the yolk retainers when going back together.

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63 Valiant Wagon
225 - 4 bbl

A 'missing tooth' in any of the rear end gears will give you that feel.
Get a gasket and pop-off the rear cover to inspect all the gears carefully.
(any broken pieces will usually drain-out w/ the gear oil)
DD