Your logic makes sense for the pinion riding up when heavy into the throttle. I don't think thats my problem because a year ago I had a 4 link suspension with all rigid steel heim joints. The pinion couldn't move up or down at all and I experienced the exact same vibration. I put the leafs back in so I could go through state inspection easier.

 

I talked to Mike at Denny's Drive Shafts again today. We went over my problem in more detail with the exact dimensions of my truck. He says they see this problem a lot with pick-ups and Chevelles due to their long wheel base. The problem being, longer shafts will reach their critical speed at a lower RPM, compared to a much shorter shaft will reach its critical speed at a much higher RPM. He is convinced I am reaching a natural resonant frequency, because I can drive through it. Above 130 mph the vibration is gone and when I slow down the vibration comes back until I'm below ~90. He assured me that if it was a pinion angle problem, I would feel the vibration at a much slower speed. Long story short... I ordered a 4 inch aluminum driveshaft from them. They will spin balance it beyond what my truck is capable of. Its 3 weeks out, so I will update this when I get it installed. In the meantime, I'll just keep it under 80.

 

goodness, well i guess thats good news....is there anyway to break this down, critical speed is determined by driveshaft revolutions, the tranny gear ratio and rearend ratio should affect this speed.....I would imagine a 4:11 gear ratio in a 4l80E in overdrive would be spinning the driveshaft pretty hard.....Any one wanna do the math for a 4l80E with 3.42 gears? What speed will i hit critical speed of the driveshaft???

 

I found the following on Car Craft.com
Other than balance, the length and diameter of the driveshaft directly affect the performance of the unit. Critical speed is the rpm at which the driveshaft becomes unstable and begins to bend into an S shape. The longer and smaller (diameter) a driveshaft is, the lower its critical speed. Critical speed is felt as excessive vibration that could cause the unit to fail. To calculate the critical speed, the length, diameter, wall thickness, and material module of elasticity must be known. Then using the critical speed calculation formula (see the Speed sidebar), the numbers are plugged in and you will know the driveshaft's critical speed.

Read more: http://carcraft.automotive.com/10487...#ixzz1MlQJhrDW


Does that help?

 

I don't think the transmission matters. The driveshaft speed is in direct relation to the vehicle speed, and is only changed by the tire height and gear ratio.

 

i believe the type of transmission does matter. The 4l80e has an overdrive of 70% so doesn't that means your driveshaft will be spinning 70% of your engine rpm. you have to take in consideration of what gear you are in. After that, you have to factor in the tire and rear end ratio.

 

I finally found the time to install the new driveshaft. Its a 4 inch aluminum from Denny's Driveshaft. Vibration is all but gone!

 

You still have the vibration>? Mine is gone and i still have the 3 1/2" Steel running in the 10 second 1/4 mile range with no problems and no vibration......Tell me you u-joint sizes from the tranny down.... Mine is 1410 on slip yoke, 1410 on front of ds, 1350 on rear of ds and 1330 on rearend, with my vibration i had a 1410 on slip yoke 1350 on front of ds, 1350 on rear of ds and 1330 on rearend....Not like this matters but im just curious, i still use the stock cross member my truck come with and i had to make a 1" spacer to go under it also

 

My vibration used to be scary bad... it is greatly reduced (90% gone). I still get a slight vibration at 100 mph, but its not bad and I feel this is a HUGE gain.
My U joint sizes are all 1350 at the slip yoke, shaft and rear yoke.

 

So you had a new shaft made? what solved your vibration? Was it the new shaft or raising the tranny 1 inch?