I have been experiencing similar situation (have only owned the bravada a few months) - did the fluid changes and drove but could still feel the added drag of the transfer case engaging the front driveline at all times.
I recently Pulled the "4WD" fuse on my 2000 and Instantly noticed that it drives 100% better exactly as I would expect for 2wd - I suggest you give it a try. It at least suggests that the transfer case motor is working (not stuck).
Now I need to find a service manual to better test the AWD system. I do not have any trouble lights on. So now the question is why is the transfer case being engaged. typically an AWD system is engaged when the sensors detect a difference in wheel speed or front/rear drive shaft speed.
I am hoping someone who has GM experience with these cars can chime in with some diagnostic insight.
be sure to check the basics first, tire pressure, and tire size/wear. a problem with one tire can cause a difference in front rear axle rotation due to the reduced rolling diameter of the tire with low pressure. Are all 4 of your tires the same condition?