Shaky/Vibrating feeling on 6th and 7th gear?

1999 Mack dump truck. So everything else is normal. I start shifting up normally, I get to 6th gear, as soon as I start getting to the end of 6th gear, there’s a very slight vibration/trembling. You can also feel it in the duration of 7th gear. 8th gear it’s very light but not like 6th and 7th. Have no idea what it is. The truck doesn’t feel as smooth like it does in other gears, at high speeds it feels a bit “loose” if that’s the right word. When driving on normal roads, the truck feels like it’s driving on a bumpy road full of small holes. And it feels terrible on roads that are actually bumpy and full of holes, not as easy and smooth as it felt before. You can feel the tremble whether it’s loaded or unloaded.

 

I was just in the shop a few days ago as well, new kingpins on all lift axles, 2 new hub oils seals, alignment, new bushings on front and in all lift axles, truck has new tires all over. I can also add, a carrier bearing was replaced a few months back and I think from that time is when the trembling started. But I’ve checked the drive train and the rest of the carrier bearings and they seem fine. After I did the kingpins which was a week ago, the truck felt really smooth for about 3 days and then this trembling came back.