Transmission slipping

I was wondering what could be making my tahoe feel like its slipping in and out of gear. I have a 1999 chevy tahoe with the 5.7 liter 350. Its a 4 door and 4x4. It has 176,000 miles on it. I recently had the transfer case rebuilt and it has been running great until about a week ago. I noticed that when im cruising through the city I live in that the tranny feels like its slipping in and out of gear. I dont notice it when im on the highway unless I gas it hard to pass somebody. I mainly notice it when im going under about 55 mph. Is there anything I can do before I have to send it to a transmission shop? I just spent 2000 dollars getting my transfer case rebuilt. I dont wanna have to spend that kinda money again if you have any ideas. Thanks for your time.