Transmission Slip

I have a 1990 full sized Chevy Blazer 4wd 350 5.7 liter. The truck has 101,000 miles on it. Recently the transmisson feels as if it is slipping or chugging along. It is very bad in overdrive and getting worse in drive. It shakes the entire truck. Any uphill resistance seems to trigger the problem.The fluid in the transmission hasn't been replaced since the truck had 30,000 miles on it. I've been told not to change it because it may cause more problems. I feel as if I already have problems, what have I got to lose? Any info would be appreciated

Thanks