Looking at a 5.5" solid black aluminum 1/2 x 36 barrel shroud, I see most of the posts and advertisements here are for 1/2 x 28. What is the difference going to be and will the 36 thread work?

A 1/2x36 device will NOT thread on a 1/2x28 threaded barrel. The pitch of the threads is obviously not the same.

Why are you looking at something with a 1/2x36 thread? There are dozens of places that sell shrouds threaded properly for the 15-22. Tacticool22.com is a good place to start.

A friend has one he was going to give me because he planted a money tree in the backyard and has unlimited funds to buy stuff!! Didn't want to even attempt to try it if it would mess up the barell threads.

Trying to screw it on will destroy the threads on both the barrel and the shroud.