9" with limited slip and 4.11 gears

[Roadrunner351w]

I replaced the gears in my diff with 4.11's and now she runs like crazy except when I am going over 100mph then there is this awful vibration in the rear and only when it is over 100mph slower speeds no problems at all. Never noticed the problem before. I change there rear gears from 300's to 411's because the 302 boss motor did not act well with the 300's but now she acts right except for the viration at 100+. Any help would appreciated.

[68 Ford Stepside]

Are you sure it isn't a u-joint or a center support bearing ?

[Roadrunner351w]

U joints are new and it is one piece drive shaft

[basketcase0302]

Were new bearings put into the assembly while the new gears went in? How was the backlash pattern? Setting up a ring & pinion on a 9" is not for the timid-it has to be dialed in with shims to get the right backlash:
http://www.youtube.com/watch?v=dAqAqODmcj4

[Roadrunner351w]

No, new bearings were not used but that is what I am leaning towards now is replaceing the bearings and redial it in and see what happens.

[69timemachine]

I'd say be sure to check your driveshaft for missing counterweights. Also make sure it has no dings or damage to the tube. It's spinning about 25% faster now, so a small imbalance is more likely to show up now. If it persists, you might want to have it tested and rebalanced. I'm assuming it has never been shortened and that the yokes are in proper phase? Also, is your transmission tailshaft bushing in good shape? No slop when you pick up on the front driveshaft yoke? Just some things to check.

[averagef250]

85 was about as fast as I ever drove my '70 F-250 4x4 until I put an NV4500 in. Had all new drivelines made, but broke the speedo cable during the swap somehow. Anyway, wasn't too used to the overdrive and got to showing off the 390 a bit on a country road with a friend in his RX-7. I guess those things shut down at 125 or so and I had no idea until I did the math off the tach that the old Ford was cutting through the air around 138 MPH on BFG muds. Those new drivelines were nice, but I think I like I like a little out of balance to keep me in check.