I am a pretty good salesman but I suck as a mechanic.
1986 f250 4wd 6.9l diesel c6 transmission
When I drive with the hubs locked in, it makes a horrible noise, unlocking the hubs silences it immediately. I think one of the u joints in the axle is bad but I am having trouble figuring out which one,
How is the best way to determine which is bad?

 

Lock in one hub at a time?

if you get as far to do one joint on the passenger side may as well do the other. I’d do all three and be done with it.

 

The frontend has 5 u-joints and two slip joints. With it out of 4x4 and the hubs unlocked, just visually inspect them, and wiggle them around and check for play. There should be no play. There will be a u-joint out at each wheel, and another u-joint and slip joint on the pass side of the front pumpkin. There will be two u-joints and a slip joint in the front driveshaft. Grab those slip joints and shake them, they are known for wearing out and causing a vibration also.