Edit. Just now able to watch the video. Definitely not ujoints as they would only do it as the load is put on and taken off.
Could be cracked flywheel or a loose converter bolt.
Definitely engine RPM related.

Don't know enough about the internals of a trans to say its anything inside but I'd definitely start with the converter bolts.

 

And while you have the inspection cover off, look at the starter. See if the starter gear is loose on the shaft, like the bendix' spring is broke. Or the starter gear is too close to the flywheel and needs to be shimmed. This might show up when the rpm drops so low in gear.

 

torque converter?

 

Ok still havent messed with it latly do to rain. Hope to take a good look at it in the morning. Any one have a second opinion?

 

Updated with a new video.

 

Planetaris or sun shell. Only seems to be when you put it in gear. If it was a tourque converter its usually the other way around. and a fly wheel would also be heard at a stop. ask a tranny shop what they think....

 

Whats the experince level on swaping those parts?

 

Highly experienced/transmission specialist. You have to pull the trans and tear it down...slightly more involved than a shift kit with directions and a video

 

I am not a trans expert but these are just some of things I have come accross. If you just take it apart in assemblies and replaced the damaged parts it should not be to hard... But if your not comfortable doing a transmission I recomend you find a good builder. I would recomend getting a book on it like an ATSG book or a factory manual. I took every last part out just to get an idea what was inside and to get everything clean. I also wanted to check every part inside and out, but I have not gotten around to actually putting one back together

 

Good advise! Since your asking I would asume you have never done any work on a transmission.