2002 Chevy Trailblazer Transmission slippage

Thank in advance for your inputs. Some time ago I repalced the Transfer and two weeks later I started to experience some problems with the transmission. When I put it drive I have to accelerate a lot in order for the car the move and when I put in in reverse sometimes it wil not couple and won`t move. The local mechanic has told me that maybe the oil filter inside the carter is dirty and plugged. Can this be true? Recently I had a problem with the oil measuring rod and found that the plastic plug inside the tube that houses the measuring rod was broken. The mechanic dismounted the carter and took off the broken plastic. Replaced the carter and put back the oil he saved. The car worked for some days and then the slippage problem began. Can you please give me some insights? Thank you