Since you refer to a main 'manager' I assume that he's not the ultimate boss.
If it was me, I'd send the main manager a (very) carefully worded e-mail. This most definitely wouldn't be on the lines of "I wish to complain about . . ." .
Instead, I'd be (apparently) apologetic: "I'm sorry to have to write to you but I think that it's my duty to make you aware of some ill-feeling among some of my colleagues . . ." I'd then go on to explain the problem but I'd take extreme care not to appear to directly criticise the lax manager. Throughout the e-mail I'd repeatedly stress that my main concern was to try to create a happier working environment within the company but, at the end, I'd add something along the lines of "I hope that you'll understand that I consider that it's my duty as a conscientious employee to copy this e-mail to X" (where X is the 'main manager's boss!)
I think that this is probably the best approach but only you know the personalities involved.
Hoping that this might be of some assistance,
Chris