Back fire? That's a classic for burnt valves. High HC indicates poor burn which is usually the result of poor compression, which is generally
the result of leaky valves. High Nox can be a malfunctioning EGR circuit or the catalytic converter is past it's prime. Compression check will answer the HC question and a cleaning of the EGR components may help the Nox.

'89 P'up, 2.6 I-Tec, 488,000 miles and done... gone to the great beyond