Not sure what isokinetic axles are (perhaps you mean homokinetic?) but I can tell you about annular. I've never understood the forum reluctance to installing them.
With a cheap Homokinetic axle, you're stuck opening the diff to replace it. Not so with a cheap annular axle. Totally mitigates the risk of using something other than a $400 a side rebuilder.

RE: installation, I encountered no issues.
The spacers already in the diff were adequate. Certainly no need to separate.
The one thing I'd suggest however, is to retorque each bolt connecting the two pieces of the unit(axle to flange?). After a few months of driving mine were almost finger loose. I noticed a thin spray of grease at the axle union. I'm fortunate to have caught it when I did. No issues since, though I do check them periodically.

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1981 240D 4sp manual. Ivory White.