Moving objects in the X and Y plane only; leaving the z coordinates the same

Greetings,

I want to move a large 3D steel file on top of a 2D architectural background. The ISSUE is that when i move it (in plan view "aka top view") the X,Y coordinates change AND the Z coordinate changes... But, I want the Z coodinate to stay the same :(

I wrote a LISP file that doesnt seem to work (SEE BELOW): can someone help me with this file?

(defun C:MoveXY (/ PT1 PTB PT2)
	(setvar "CMDECHO" 0)
	(setq PT1 (getpoint "\nSpecify Base Point or [Displacement] Displacement: ")
		PTB (getpoint "\nSpecify second point or <use first point as displacement>: ")
		PT2 (list (car PTB)(cadr PTB)(cddr PT1))
	)
	(command "_.MOVE" (SSGET) "" PT1 PT2)
)

 it seems my issue comes when i get to the command line (ssget) seems to mess it all up :(

Can someone please explain how to use the ".Move" command with the (ssget) function?

Thanks