2002 Chevy Blazer Possible transmission problem??

I have an 02 chevy blazer 2wd, v6... Recently when hitting high speeds (50-60) mostly going uphill... It feels like it's not shifting or something. I for some reason have trouble getting past 50-55mph and kind of a pinging or rattling sound is coming from under the hood. I just can't get it to get over 55 uphill at all... The RPM's are reving when I press down the gas, but I'm not getting anymore speed and the temp will move up slightly in doing this. Is this a tansmission issue and if so, what can be done? car has always run great, but then this. I know it's not a fuel pump as I just got a new one at about 60,000 miles. Can you think of anything?
thanks