dog walking/bent trailing arm?

[rodnok1]

My 64 Panel has always dogwalked just a little, wasn't able to locate anything out of wack until recently. After hauling a very heavy trailer recently it started tracking worse. Frame is square, I used the shop manuals to confirm. Front end in good shape, needs rebuilt, but doesn't wander with new steeeing box/pitman arm. I checked the wheelbase and it is different almost 3/4 inch from left to right. Left side is almost 3/4 inch shorter, checked it 3 times to be sure. I cannot seem to isolate it to trailing arm or maybe bent mounting point. Has anyone had this problem, the arms do not appear bent(i am going to template right one and use to check left soon) The other question I have is is the rear end held in line with just the u-bolts or are there alignment pins. rear axle is not bent also. Maybe the left side trailing arm has slid out just a bit?? The ubolts are tight, however it does look like the left bracket is not setting 100% flat on trailing arm, might be up just a tick in frt...hard ot say with lots o crud. Any hints on checked rear susp would be great.