Vibration at Lower Speed W900

I bought a truck about 2 years ago. 2006 KW W900L(850,000 miles). C15 with and 18 speed. 4-bag flex-air suspension. Mainly using it for hauling silage out of fields and general farm use. Put around 25,000 miles a year on. When i bought it it was it pretty rough shape and i had the truck day cabbed and the frame shortened. Went from 265" to about 220". Since i have got it i have had a vibration around 20-25 mph. It is the worst it seems like in 4th gear. Can feel it slightly in 3rd and 5th as well. The harder the truck is pulling the worse it is. Also seems like it is worse when the power divider is engaged. Last spring i had the passenger side front axle out of it and the inside splines were very worn. In the mean time i have replaced motor mounts, clutch, carrier bearing, u-joints, etc. Ride height is correct. Am i right to suspect the front differential or power divider? Any ideas?

 

Any ideas?

 

Yes.....but from the shop that did the work. They are a well respected place that does all the driveshaft balancing and shortening in house. They verified all the angles but i am thinking about getting a second opinion before digging too deep into everything else.

 

Pulled the plug on the front diff. Dirty as hell and bunch of metal on the magnet. Dropped the power divider....shes junk. Hopefully a rebuilt power divider will fix it.