727 - Let me describe my symptoms...ugh

Your reverse band is grabbing it sounds like. Reverse and 1st use that band I believe. It "locked" up on you when it tried to shift to 2nd and the reverse band was still engaged.

I had my 727 rebuilt last year by a "professional" trans builder that specializes in mopar. That POS rebuild made it about 5 miles before it lost 1st gear and slipped through 2nd. Reverse was also gone.

He re-rebuilt it as a warranty. He blamed my new PTC converter, new pre bent cooler lines, or my new HD cooler as the issue. The 2nd build made it a few more miles (about 10) but it locked up on me at ~35 MPH on a backroad and actually locked the rear wheels up! I spun off into the grass and had to get towed home.

I pulled it apart and the reverse band was locked on and had destroyed the drum. The servo was cracked and bent. I think he messed the valvebody up on the 2nd rebuild and caused most of that.