trailor sway

Well I'm not sure what is causing my trailer to tail swing but it is any wind or trucks it sways as I said my last truck was a 2004 f150 4x4 and I didn`t have any problem I set the WD to this truck like I did the old one but it didn`t help I put a antie sway bar and it helped a little but it didn`t stop it. The sticker on the trailer says 8700 pounds dry the truck sticker says 6500 it pulls it fine my tires are I.T. my trailer says the tires should be at 50psi but the tires say no more than 30 this trailer had not been used very much when I bout it. we have only run it on one short trip and it was fine we have been doing most of our traveling with motor cycle. I do think the truck is to lite but this weekend I`m going to scale it and go to a local dealer and see what they say.Again thanks for all the advice I`ll let everybody now what happens.