What causes auto to pop out of gear when stopping quick then popping back in?

What is your question?

I am looking to buy a 98 t100 4x4 187k. All looks good except that when braking quickly, truck pops out of gear. Pops back in when you start back up. Owner said needs a new silenoid. I am a little worried since I do not want to get into a big tranny job. Any help you could offer would be greatly appreciated. I plan on taking to dealer to do a prepurchase but want to know if I am just wasting my time?? Thanks!!