As far as knowing what axles you have, it is almost impossible to know. Even if you pop them apart, you will know if you do or do not have the tripod joint. As far as larger/smaller CV issue, it is not an easy thing to do unless the parts are side by side. Outside of the original Isuzu shafts, there are all kinds of joints and shafts (like the tripod joint) that are used by rebuilders.

If you do get new (or reman'd) shafts, then I suggest you replace them as units. Save the old ones and re-boot them if you need spares for wheeling, or see if you can pawn them off on someone looking for the same.

In order to use the threaded hole in the end of the shaft, you need a bolt that is the correct pitch and size. You also need a washer that is big enough to go over the end of the axle and rest on the center splined part of the hub. The bolt needs to be tightened until it just sucks in the shaft and it seals against the back of the knuckle. It is a hit and miss, trail and error kind of thing. Once you get it, something like Locktite would be a good idea as the bolt will not be under a constant torque load and could back out.

Good Luck,
Michael