2006 CM870 vibration in floor 6th gear

[berrytrucking94]

Currently having a vibration in the floor that continues to get worse. Only happens when throttle is engaged, under load up until 1600rpm then seems to even out. Mostly happens in the high side of 6th gear, some on the low side but not as bad (18 speed). As time goes on I’m noticing it very slightly in 7/8th around the same rpm.

Recently replaced the yoke and nut at the tranny because the big nut loosened up and was causing some vibration in the driveline.

2007 Pete with an 06 cm870. Truck has just over 423k on it. Valves adjusted a few months ago, driveshaft/u joints are all solid..

Any ideas what it could be? Vibration damper? Bearing in the tranny?