In SketchUp, this is the blue Z axis (vertical.)
With nothing selected, activate the Move tool. Click on the top corner of a leg that needs to be raised. (I am assuming you have grouped your legs, or they are component instances. If not, you should group them.))
Move up or down until you see a dotted blue vertical axis appear (running through the point you had clicked,) then stop moving the cursor. A moment later you should see a “On Blue Axis” tooltip appear.
HOLD down the SHIFT key on the keyboard to lock this first inference. You’ll see a bold dotted blue inference line overlaid upon the lighter vertical blue dotted axis.
With the first inference locked (still holding down the SHIFT key,) move the cursor to a corner of a higher leg (or any other vertex of any other edge or face in the model,) and you’ll see a second suggested inference popup which reads “Constrained on Line from Point”. Click this vertex while still holding the SHIFT key.
Your leg object will be moved vertically up to the constraint line level with the second click.
You can move multiple objects, in this same way by selecting a set of them (with the Selection tool,) before choosing the Move tool.
You can move objects this way constraining them to whatever axis you need, not just the blue axis.