question about rearend

Hey guys. So here is my dilemma. I have a 2000 FLD120 truck. Vin is F02319. It was originally a twin screw but at some point someone chopped it down and made it a single rear axle. It should have a meritor airliner 20K rear end in it. I purchased it a little over a year ago. It is setup as a mobile home toter truck. The guy I bought it from who I also run with from time to time said he had been putting a differential in it about every year. He owned it for about 7 years. But since I have owned it I have already put 2 in and its about to go again. Diffs have been done buy a local guy and the shop I normally go to replaces it under warranty. But this time the rebuilder is refusing because he is saying that the 20K rear that is in it is to small to handle what I am doing. I don't know that I agree with that as I know guys that have a 20K rear that don't have this problem and haul the same trailers I do. Anyway, the guy at the shop and the rebuilder are both saying I need to upgrade to a 23K rear. Shop guys says they have done it with others and it has solved the rears going. So my question is, do I need to replace the entire rear end or can I just replace the differential and the axles to upgrade it? One place that's a used parts place that has the upgraded diff and axles says I can and the guy at the shop says I need the whole rear end. Obviously if I don't have to replace the whole rear end then it is less work than just replacing the axles and diff. I am asking because I think I am going to tackle this on my own since the shop wants like $3000 to swap it out. Any input would be appreciated. It did have a slight vibration especially going up a hill loaded. By the way, dont know if this was causing issue but last week I found that the rearend was not straight. It was 7/8" off. Meaning the drivers side of rear was towards front end and other side opposite. I straightened the rear out and put new U bolts in and have not had any vibrations since but I believe damage was already done to the rear.