When you go into lockup do you have full lockup or that partial engaged PWM bull crap that lets the converter slip a little? I know on the half tons that have this they reach full lockup at around 55 mph but a 3/4 ton may be different. Sounds to me like you are getting some converter clutch shudder. I've replaced two converters because of this. One was in a 05 half ton Sierra and the other in a GMC Acadia of all things. On both I could watch the slip speed and when it started the shuddering it would be all over the place as high as 150 and as low as 0. Once it hit full lockup it held fine and there was no more shuddering. At the speeds you are saying I think this may be kind of a stretch but its worth looking at if you have the scan tool to view your slip speed.

Are you sure you don't have an ignition related misfire? You usually get those right around 55-75 light load cruising speed when the mixture is kind of lean and the load is kind of high like pulling a hill and that weak little spark can't quite make the big bang happen.

 

Any resolution for this yet?

 

what did you find out?

 

no vibrations in 1st or 2nd . . . .

I only get the vibrations when it's locked in 3rd and 4th

I ordered the valve body repair kit, but I ended up getting the refill kit vs the new drill/reamer and new valves . . . I'll have to wait for another week.

 

Had the same problem with my 2500HD 2 years ago and it ended up being a bad converter. Replaced it with an OEM spec replacement along with a new filter/oil......still going strong with over 200K miles.

I'm not saying this is the case but you might want to double check before dropping a lot of coin on more parts.

-Isaac-

 

I have the same problem with 4L80E - when I lockup at around 38-40mph or below, I hear loud noise/vibration, almost like smth. is grinding. It goes away above 45mph. Converter is Circle-D multi-disc.
Any suggestions?

 

The vibrations went completely away after redoing my injector pigtails. I had one or a few bad connections on my injectors.

I still have a clanging metallic dragging noise that's going on while in park or in neutral. The oil is still clean, even after I got my truck back from being stolen.

 

Interesting... and you weren't getting any misfire codes? Nor did it happen when you werent locked? It's amazing we ever figure out whats wrong with our cars lol.

 

I ended up getting pissed off enough with it one night after a long road trip and started over with a stock tune, it helped that I borrowed a wideband and several emails to Jake Brown and to Wheatley

Needless to say I was embarrassed that it was as simple as an injector adapter

 

Mine makes the metallic clanking noise in park/nuetral as well.... been doing it for about a year now with no other issues. Dont know what it is.