I believe that OP is talking about outside central air units. I couldn't
fathom window units not being level without window closure problems.
I have been before and can be wrong now, but that is what I get from it. I
don't think level is critical here although leveling them really shouldn't
be too much of a problem. A pry bar and 4x4 piece should help raise the
desired side, them pitch in some sand and recheck for level. I am assuming
that the unit is sitting on a concrete slab. If not the "feet" can be
levelled by using appropriate concrete pavers. Be carfeul not to rack the
frame OR twist/bend or mishape the freon lines, you may not have alot of
play with them so be careful.