This is strange, because my truck does the exact same thing...and I was pretty sure it didn't when I went to look at it to buy it.
I thought that it started becausewhen I bought the truck, it came with a spare 6.2l engine and spare th400+tcase. The old owner loaded all this into the back off the Blazer when I came to pick it up, but he owned an engineering business andput them in with a forklift.
When I got home, myself and and a friend got the gearbox out, but the engine was to heavy. It was a few days before I could get time to rent an engine hoist, so I was driving round with this 6.2 engine in the back...every time I gave it to much gas, the engine rolled back and hit the tailgate, brake too fast, and it would roll forward...I drove pretty carefully fo those first few day. Anyway, the engine left a dent on the inside if the tailgate door, and I assumed that this dent was causing the mechanism to catch, I just haven't got round to having a look yet...