I just lowered mine on purpose to make it pull my 5er more level. Newer trucks are way too tall!
Was it lowered by replacing the springs, so-called "flipping" the axle, or by replacing shackles and hangers? Replacing the springs with lowering springs would inevitably lower their capacity, but the other two options retain the full spring capacity and would not really be of concern. Obviously use a proper hitch for the weight you're pulling, which would be a receiver and not the bumper. Other than that you'll probably be fine.