Still can't diagnose vibration
So my rig has had a vibration since I got it, and I still haven't been able to track it down. Here are the conditions. At first it was a narrow rpm band, about 2700-3000 rpm where it would happen, cruising. It sucks, because that is right where I am at when towing with the OD off down the freeway. It runs very smooth up until this point. When it happens, it's hard to tell where it's coming from specifically. It just rattles the whole truck really. I have checked all my u joints, and while I would like to replace them, I don't think they are the cause because it is rpm related, not speed. It seems worse under load, so if I put it in manual first and drive at that rpm, it's not as bad.
Does anybody have any ideas? The only thing I can think of is potential bad motor mounts, but I don't really know how to check that without replacement. The rpm band that it vibrates seems to have gotten larger, starting at maybe 2500 rpm and not really ever going away at high rpm like it used to. It seems to run right otherwise and has decent power, expect it also has a slight miss (I think it's a miss) at idle but it smooths out as soon as I start moving. I don't think it's related and am not worried about that one really, unless it turns out they are related.
If it helps, I know that this truck originally had a 5.9 and someone swapped in a 5.2. I have no clue why, and didn't know it when I bought it. They didn't don't a very good job, so it could be something uncommon that you wouldn't think of as a wear item or anything, or maybe the motor just wasn't aligned correctly.
Any ideas would be great. I want to keep it, but it makes me really uncomfortable towing with the vibration, and I seem to be towing lot with it and don't want something breaking on me