I noticed that my truck vibrates around 50mph and generally anytime the lockup engages I think. If Im traveling at crusing speeds and accelerate slowly without the tranny downshifting the truck vibrates till the lockup is disengaged or I depress the pedal further. Im not sure what the problem is but it seems to only happen when the truck is cruising.

Its got larger tires (33's) which I assume is why my speedo is off. The truck shifts fine through all the gears and has no problems at WOT either. Don't know where to start troubleshooting?

'00 2500 6.0L ECSB

 

If the problem only happens with the tq converter locked up, I'd guess that the problem is in the converter. If the problem always seems to happen at a certain speed, I'd be looking for a tire imbalance or possibly a driveline imbalance or angle problem.

My truck had a vibration between 55-65 mph when it was on the original Firestones and also with BFG 33" AT's. After I lowered the truck via front torsion keys, removing the rear lift blocks and installing 2" rear lowering shackles, my vibration disappeared. I never checked the pinion angle, but am guessing that I either had a problem there or possibly even with the u-joints, that was cured by changing the driveline angles when I lowered the truck.

 

If you had a tune, the speedometer, shift points, and TCC activity would all get corrected. That alone may take care of your problem. If a TCC shudder is left alone(ignored) for too long, you may damage the TCC.

 

Well I started noticing the vibration a lot lately, seems like 4th gear might possibly be slipping. The truck used to throw a trans input/output speed variation code (IDK what it was off-hand) but it went away now...very weird.

I've noticed that the truck drives fine without any problem whatsoever when the trans is cold, soon as it gets to temp it starts acting up. Don't really know where to start troubleshooting....it would be a shame to pay for trans service then have to dish out more for a tuning to correct the TCC problem.

 

TCC lockup doesn't happen when cold, and the problem isn't there when cold, so something's going on with the TCC or it's control circuit...