727 lockup

I have a 727 behind a 360 in a swinger. The trans was just rebuilt by a tranny guy I trust. Its been sitting a couple months as I had to replace a heater core. I took it out yesterday and backed it out to the street put it in drive and when it got to 5 mph the back tires locked. The idle came down and it moved again to 5 mph and the back tires locked again. I went home put the rear on jack stands. I started it up put it in manual 1st no lock Put it in manual 2nd lock. Idle comes down and the wheels spin again. Drive same thing locked. A couple of things to add. when I first put the trans back in Reverse blew in a couple minuets. A little piece of metal 1X3/4X 1/8 that the reverse band pivots on broke in half. Replaced that and got reverse back but had to put it in drive first to get reverse to fully engage. Trans guy says pull it out and I'll take a look at it. any ideas before I do that thanks guys