Can a Eaton 18 speed transmission slip?

You should move this to the GARAGE page.

 

Are you air supply lines to the tranny kinked in any way or not attached properly?

 

If you had flywheel ground make sure they did not take too much off, if there is no relief groove around outer edge of surface the flywheel has been cut beyond use, this will take clamp load away and let clutch slip, even more in high gears, good luck.

 

You know a new clutch will slip for some time

 

might not be the transmission, could be a problem with your power divider..or possibly a low torque clutch was installed..

 

Did you use a factory new clutch or somebody's reman. Some remans are junk from day one. You said you tore into it. Is the clutch out of the truck where you can examine it? I had a reman one time that the piece with the buttons riveted to it was twisted. This caused partial contact of the buttons with the disc. I took a lot of flak for abusing the clutch because it needed adjustment every 6 weeks. When the installer removed it I got the last laugh. Junk clutch!

 

I have 15speed. Last year had it replaced. I could shift in to gear but felt like the clutch was slipping only in higher range, not low range. Had rear box gears replaced. Worked good for about 1000 miles and started to slip again only in high range. To shorten the story. When I got home 3000 miles later, running empty and only get up to 50mph or it would slip. The input shaft from the main box to the rear box was worn.
Every gear position I would feel it go "in", but under high torque it would slip.

 

That's BS ,A new clutch will not slip ..Most likely the wrong clutch was installed or the installation was not done right,,Need to go back to where the work was done and demand they fix it right,,sure hope he did not use a fly by night or a cheapo shop.