I just happened to be setting up my drafting approach for my updated program, and came across this looking for help on something LIKE what you are talking about. I believe I understand what you are trying to... obviously there are other ways to do it, but:
In the upper right corner of your viewport you should have ViewCube (if not turn it on), right beneath it you should have a UCS (if not.. in the ViewCube settings menu (R-Click, ViewCube Settings), there is an option to turn it on). Between these two visuals you should have a good idea of what is happening.
Likely, by standard, you see "Top" on the View Cube, and "WCS" (World Coordinate System) for the UCS.
A really quick way to manipulate your UCS to simply move something on a constaint Z axis is:
- Mouse over your ViewCube, an arrow will show below it. Click it and your view will change to "Front". (Obviously if you choose either side arrow, the view will change to "Left" or "Right".. doesn't matter)
- Next type "UCS" in the command line, hit enter. select the option "View" or just type "V' and hit enter again.
- Now your ViewCube should still say "Front" or "Left" or "Right".. whatever you had set it to... BUT beneath it the UCS will say "Unamed". Because you essentially created a temporary UCS based off your screen view and haven't named and/or saved it.
- Now you can move on a constraint WCS's Z-axis ... but... your doing it along an Unamed UCS's Y-axis.
- To get back your "regular" view. Just click the upper arrow appearing on the ViewCube... it takes you back to "Top" view.
- Then just beneath it... select "WCS" as your UCS.
There is NO DOUBT other ways to do this, pending system variables and any custom UCS, views, viewports etc... this is just quick fix way I do it.
UCS stuff can be complicated for a new user, but I'd definitely learn some of the basics. You'll realize you can save a lot of time drafting certain projects by manipulating, and saving custom UCS (USER Coordinate System)