Working on my 98 ss . top end chatters under load but only does it between 2500 and 2800 rpm. Could this be a rocker or is it definitely a lifter collapsed?

 

Engine load condition has no effect on noise created by either rocker arms or lifters. What you are probably hearing is spark knock, which can be induced by engine load, especially around the torque peak. Make sure the spark plugs are of the proper heat range and that you are using premium fuel. Using a scan tool look at the KS (knock sensor) action as it may not be working properly, and check the fuel pressure under load to make surethat FI is not leaning out.Finally, re-tune the PCM if it was molested by someone that did not know what he/she was doing.