bad gears?.. driveshaft unbalanced?...


Founding Member
Jan 31, 2002
your moms bed
ever since i put in my gears about 2 years ago, i have this harshpulsating vibration at about 90mph and it kinda stops at about 110 if i remember correctly. Its basically hitting a harmonic vibration around that speed and its annoying and sounds like its not too good for the driveline. its definately the rear. so i would think automatically its the gears, but im not sure wether it started exactly when i did the gears or what so what do you guys think. bad gears (used fms 3.73's off ebay), bad diff bearings? bad driveshaft?, bent axle? anyone have this same condition?. i bought a whole new diff install kit and im picking up some 3.08's tomorrow and i guess ill check the axles while they are out and see if they are bent. anyone?
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nah stock 94 piece. i was thinking im gonna do the gears and diff bearings, if its not that i got a junkyard with an aerostar ds for $75 . since the fms ones are getting so outrageous im gonna cheap out and hopefully get a good one. although the ds place by me only charges $50 to balance , maybe ill just get my stock one balanced. i put the rear axle up on jack stands and the vibration occurs about 130mph with no weight on it... whatever that means...
When I bought my car from the prev owner it had a similar situation, I replaced it with a aluminum d/s and problem went away, but that was when they were about $100 or so cheaper than now. If you want to save money, I would ahve the d/s rebalanced or chkd out.
I really kind think it is the endemic issue with making a stock DS spin much closer to its critical speed (with your geats), and not the gears so much.

Havin a shop dynamically balance the DS would be kinda cool (make sure they know you hit hyper speeds). The dynamic balance will take into account any other driveline imbalance.

That Ford Aerostar piece sounds nice. Is it from an AWD? I have a part number somewhere for the U joint or whatever it is you need.

Good luck.
I dunno... when I had my gears done it was done poorly and i got a horrible vibration. put in the alum ds and most of it went away. Now that the gears are insalled proper I still get a harmonic vibration starting at 80mph and it continues to get worse as i go faster. when i first ahd the gears re-done it started at 72mph or so and I typically only drive below 80 anyway so I think i jsut haven't completely wore the gears back into themselves.

Now for the ds i would say get new u-joints and a balance and you will be fine. I spent the extra $$ for alum because when i called around it was less than a $80 difference between u-joints and balance and all new.

Just my $.02