Low speed bounce

I have an old MVT truck, 2012 prostar+ with a maxxforce 13 and a 10 speed manual. truck is a 3 axle but only the rear front axle is a drive. i guess the right term would be 3 axle single screw. I have a bounce or hop only between 10 and 15 mph. it happens in 4th, 5th, or 6th gear. weight aggravates the problem. i can't make it happen bobtail. empty trailer it happens but not terrible but it is there. loaded it is atrocious. flat ground i can ease through that speed range but any incline taking off it just shakes the truck. as soon as above 15 mph the truck is smooth.

differential yoke and u-joint are new, carrier bearing is new. not sure where to look for this issue or if it is common.

i run the pacific northwest. any help figuring this out or recomend a shop that would be good at figuring this out.
TIA

 

Yes. The axle lock switch dumps the air in the dead axle when engaged.