insane vibration!

[jansen]

First things first, I got a 99 rcsb with the 4.8 and 4l60e. I've got a vibration at 90 mph. Its a really fine vibration but it's so bad it feels like your driving a womans sex toy! I work in a driveline shop and I fix vibrations for a living but this one has me stumped. The vibration is so bad that when we set the truck up on jack stand in the parking lot to search for the vibration it was so bad the driveshaft was flexing in the middle until it broke and went flying across the parking lot. I replaced every bearing in the rear end last night and chucked my axles up in the lathe to make sure they were straight. the new driveshaft that I built is a lil beefier but is the most perfect shaft I've ever built. Also replaced the bushing on the tail end of the transmission and crossmember..... Still a terrible vibration!!!!! Also some times at slow speeds under slight acceleration I hear almost like a poping or binding, it almost sounds like something rolling around in the back of the truck but its not. The truck has 200000 miles but the tranny has about 10000 on the build. Do you guys think this is tranny related? Seems like that's the only thing it could be! Prease help!

[RedHeartbeat]

have you checked from soft, broken or worn engine/transmission mounts?

[jansen]

have you checked from soft, broken or worn engine/transmission mounts?

Replaced the trans mt already and the engine mts I thought looked good, gonna check them again!

[ducky]

What tires are you running? When I was running drag radials on 7" wide rims in the back, they were over inflated and would shake the hell out of the truck at like 60mph

[jansen]

Yokohama avid sub 275 55 r20
trying to sell these 20's and buy some dr's on 16's.

[Slammed96]

Have you checked your pinion angle?

[jansen]

Truck is factory set up, no drop.

[jansen]

Well..... i think the problem revieled itself today when my rebuilt 4l60 with 10 to 15 k on it took a dump on me!

[skinny chicken]

Glad to see you found the problem but it sucks you killed the tranny. I think GM made the 4l60's out of popsicle sticks and crazy glue.
The best of luck to you with the rebuild or swap.

[Botlfed]

doesnt necessarily mean that was the problem, could be something else that caused your trans failure, possible warped flex plate, broken motor mounts, somtime theyre hard to see especially with the motor in, also converter bolts can back out believe me. dont just chalk it up to a bad trans, check everything, or else youll end right back in the same crap youre in now.