turbo 400 w/brake locking up in 3rd

[69trk]

Drove the truck today in the neighberhood, and went into 3rd gear and the transmission locked the rear wheels.. Good thing i was only going about 20 mph or so... What would cause this? It has a wilson's turbo 400 with a transbrake. I ran it alittle low on fluid when i took the driveshaft out. So i added a qt and it's still locking up in 3rd.. As if i'm pushing the brake button.. Could it be the converter, shifting linkage, or brake? I didn't feel like working on it today, so mabey i'll crawl underneath it tomorrow..