just replace the cv axle on my truck, the problem now is that the cv axle will not lock into the shaft coming out of the front diff. Any suggestions as to why it will not lock in. When I drive it on the road, the truck shakes significantly, and its looks like the cv axle moves left and right along the shaft that comes out of the front diff.

 

I believe there's a "snap ring" in the front diff, that locks the driveshaft into position.
The drive shafts can be a pain to get out and at times so bad, people can dislodge this snap ring and it falls into the bottom of the diff.
Did this happen to you?
Were your CV shafts so bad you had to use a slide hammer on them to get them out, or other such heavy duty methods?
It's something to check.
Al.