4 wheel drive

The only thing I have to go on is the pictures in the parts book as I have never had any issues and never had mine apart.

Some questions:

What make model and year of tractor are we talking about?

You asked what is the root cause of the failure but what is failing?

Are your circlips breaking or slipping out of place?

Other than replacing the balls with slugs (I think this is a really bad idea), what other parts have you replaced?

Did you use OEM parts, after market or something else?

Is it always the same joint that gives you problems? Which one?

Based on the drawings in the parts book for my 2006 Jinma 284  here are the possibilities that I see.

Using the wrong clip would cause it to either break or not stay in place. These things are metric, and off the shelf imperial won’t fit properly.

Groove that the clip seats into is worn or damaged. In that case even the proper new clip won’t help. The shaft has to be repaired or replaced.

The splines that the balls engage against are worn. Using slugs (cylinders) instead of balls will cause damage to the splines as they don’t allow the full range of motion intended.

The same thing applies to the splines inside the coupler, they could be worn too. It might be that the wear is evenly on the shaft and the coupler, but when added together the wear exceeds the service specs.

This is a constant velocity joint, what type of lube are you using in it?

When it is assembled there should be very little to no rotational free play (backlash) in the joint. If there is too much play, it is worn and can not be expected to perform properly.

The spring may not be exerting enough force on the joints to keep them pushed together during use. Or too much force could cause the circlips to fail or slip out of the grooves.

The fine tooth splines on the transfer case end of the shaft must slide freely in the coupler or it could cause binding that would put undue stresses on the shaft and joints.

Without seeing the problem we can only guess at the cause. Please post some pictures, maybe we can figure it out.

is the tube that holds the driveshaft straight? At one time I high centered the tractor and bent mine up pretty bad and I believe if left alone would have caused issues with the coupler and said balls.

I made the ball to slug modification 1000 hours ago and have had zero problems.
Absolutely easy shifting transition from one wheel to 4 wheel drive when engine torque is lessened/relieved on drive train. I did chamfer all slugs on both ends.
I consider this an excellent modification and need to again thank the CTOA member whose tutorial I relied on.