Reverse is sluggish

Recently bought the car. It shifts into reverse just fine and if you do not push the gas it rolls okay in reverse under its own power. When you go to give it gas it is like it is slipping and struggles to move. I noticed that the linkage that connects the four wheel drive was missing. I found one and installed it but the rear tires would lock up and you can really noticed it when turning. I am unsure if these two problems have something in common but just thought I would add it. Right now the linkage is out and the reverse problem is still there. What could the issue be? Thanks in advance.