OK, well, since there doesn't seem to be to many known solutions for why the front drive shaft would disengage in 4lo, I pulled off the boot and crawled under the rig to have a look-see.
I noticed when in 4lo, if the shift lever can be move foward about a 1/2 inch, this will disengage the front drive shaft. There is a bracket behind the shift lever, so I loosened the bolts, pulled it foward, and re-tightned. Now, the shift lever cannot move foward, causing the front shaft to disengage when in 4lo.
Not sure if is the 'real' reason it was popping out, but, I think this should prevent it from happening.
I noticed when in 4lo, if the shift lever can be move foward about a 1/2 inch, this will disengage the front drive shaft. There is a bracket behind the shift lever, so I loosened the bolts, pulled it foward, and re-tightned. Now, the shift lever cannot move foward, causing the front shaft to disengage when in 4lo.
Not sure if is the 'real' reason it was popping out, but, I think this should prevent it from happening.