Noise when manual hubs are locked and in 2wd

I have an Umm Alter 4wd truck with manual locking front hubs. I use the 4wd routinely, and so have often left the hubs locked while driving in 2wd. Recently, when doing this there is a loud popping or banging sound coming from somewhere around the right front wheel or front differential. If I unlock the hubs in 2wd, this does not happen, and if I am in 4wd, it does not happen, only with the hubs locked in 2wd. I do not know much about differentials (I dont even know what style the Umm Alter has) but was wondering if anyone had any ideas. Or could it be the u-joints? (no cv joints on front wheels)? Thanks.