Vibration at 45 mph only in 2nd not 4wd

[Wellis]

Hello I’ve had this vibration since I’ve had the truck it’s a 1995 gmc 3500 dually 6.5 turbo I’ve had all tires balanced new rear drive shaft new front drive shaft bearings and new used transfer case the truck starts to vibrate at 45-50 mph and you can hear it anything after 50 there’s a slight vibration but when I put in 4 wheel drive it goes away please help I’ve been trying to fix this thing for a long time

 

[Schurkey]

Worn-out front suspension/steering?

Worn out front differential?