Some slop is going to be normal wear.
One thing that often gets overlooked is RPM.
It does not take much over the normal idle RPM to make a lot of noise when you put it in gear.
If your base idle is off, then it needs to be adjusted.
Otherwise, every joint, gear, and spline will have some slop. It all adds up.
But, one may have more than normal.
If the rear output splines are worn, and you have to go in there anyway, ask around about a slip joint eliminator kit. Don't know if its available for yours or not.
The main advantage I see, is that it moves the spline out of the transfer case into the drive shaft where its easier and cheaper to fix when it wears out.
If the RPM is good, then try this:
Chock your wheels good. Put the truck in Neutral, slide under and grab your drive shaft.
Twist it rapidly from one stop to the other listening for the noise and looking for excessive movement in U-joints.
Once you hear the noise, if you cannot isolate it, try twisting the shaft more gently to see if you can run it down.
Also, push it up and down looking for slack.
The problem could be in the rear end. The rear yoke should not have any up and down movement.
If you have a locker, lots of them have more twist slack than a regular one.
Make sure you inspect all mounting bolts and nuts really well. Spring bolts on the rear end, transmission mounts, transfer case to transmission bolts, anything in the drive train that holds any part to the truck or to another part is suspect.
If all else fails, drive it until it breaks and then let us know what it was.........
