Why does my engine die when I put the tranny in gear.

This has been a good truck for the most part. When you first get in it in the morning, it's kind of sluggish, I thought of it as a fuel problem, but I have been all the way through the entire fuel and intake system with no good results. For the past week, it has been getting worse along with a vibration when shifting. Finally yesterday while at a stop light, I went to take off and the engine died. I put it up in neutral and started it right back up with no problem, but as soon as I put it in gear it died again. I was able to limp it home and get it up on jack stands. If I start it up and put it in gear without any resistance on the rear wheels it's fine, but as soon as I apply the brakes and slow the tranny down it starts vibrating violently and dies. I have removed the pan and drained the fluid and nothing seams to be out of place with the exception of the fluid looking very thin and a little too dark. Would a malfunctioning torque converter cause this? Should I try to replace the fluid and screen, maybe try some Lucas tranny conditioner, or something like that? Or would you lean more towards replacing the tranny all together?