Auto Trans Binding In 3rd

[Scottys84]

Hey guys, I recently bought my Jeep, and after getting it running and rebuilding the steering column, I was finally able to actually drive it. So far I have only driven it up and down my street. When it shifts into 3rd gear, it feels as if the brakes are applied and the rear wheels start to lock up. They almost lock completely up such as skidding enough to leave dual black marks on the street. Then it releases and coasts when it gets down to maybe 5 mph. Any ideas on what's causing this? Thanks much!
Scott