06 w9 drone under load

Alright,new to this.ive got an 06 w9,very recently converted acert to a single turbo/6nz setup.truck has 580k on it,18spd,3:55 drives.when i first got in the truck and got a wagon behind it i noticed a bad drone in the cab.i dont think its an exhaust drone (truck IS straight piped,but were all used to that)in all gears between 1350&1600 rpm.ive replaced motor mounts,front cab mounts,shocks all around.ive checked the entire driveline from the yokes to the ujoints,no slop in the input/output bearings in the rearends.no slop in slip joints.ive crawled up it a hundred times looking for something rubbing on something under load,with no luck.could this be a transmission drone?internal rearend problems,such as a bearing beginning to go?do i need to break down and buy a cheap muffler and experiment with that?has me puzzled and half deaf.any ideas appreciated

 

Yeah there is some vibration coming up through the floor,however nothing throught the shifter or clutch petal.thats why i changed cab mounts (with no change.)the drone im hearing is a constant deep groan between 1350&1600 rpm,which puts it throughout most of the useable powerband.no crunching,no chattering or anything that would indicate something loose.i think you may be right lilli,might be coming from front drive.ive pulled both plugs but found no chuncks.how does a person go about distinguishing transmission noise from rear end noise?put somebody on the catwalk with a 12pk and go for a drive?

 

Truck now has 580k on it,to my knowledge trans and rears have never been touched.truck has spent its life pulling grain and livestock.

 

Hahaha That would be an option if i were considering getting rid of the truck.to me it feels like gears moaning,but with no pieces on the rearend plugs,im back to guess work.

 

Im am certainly NOT certian of that...definitely a possibility.now we might be on the path to something,i know the only other previous owner very well,lets just say money isnt an issue.if something was wrong it wouldve been fixed.ive adjusted ride height both ways,the only thing i havent done is to make it drone and dump the air to see if changing the driveline angle changes anything