something getting loose in the drivetrain

[tree98]

It's like it isn't "catching" As soon as you roll on the accelerator after a shift. You can feel a slight clunk when it "catches" not as noticeable when the truck is loaded. All the U-joints look good. Is it common for a ring and pinion in the rear end to get worn out? or maybe something in the tranny? kenworth t-800 straight truck 265" wheelbase cm870 18 speed 46K rears