Trying to sell my ESi and the ECU is going bad....spitting out random codes, then adjusting timing accordingly. It seemes to sputter at WOT at high RPM's even before it would kick out the CEL codes, and then just get worse as the codes added up. So Mitsu determined it was a new CEL and wanted $700 for it. How easy/hard is it to swap an ECU on my own? Can I simply unbolt/unplug it and bolt/plug a new one for the same model and year up to my car?

yes. On a 95, one 10mm bolt, Unbolt the harness, put the new one in, tighten the bolt and go. nothing to it.