A 5" exhaust has surprising little to do with exhaust backpressure, at least as far as the PCM is concerned. The PCM measures backpressure BEFORE the turbo so the turbo itself is the restriction.
With that said, what I have generally found is that you will get a code for exhaust backpressure when the difference between backpressure and boost is extremely large. Normally your exhaust backpressure is 1.1 to 1.5 times your boost pressure. I am not sure what the exact ratio is, but somehwere around 1.8 to 2.0 times your boost seems to set the light. This also seems to occur predominantly on 94-97 and some 99 trucks with stock turbos and hot programs, especially with injectors. I'm certain it has to do with the A/R ratio of these turbos.
My advice is to track your boost and backpressure values. If you are seeing excessive backpressure, you might want to consider changing the inlet housing and wheel to something that is going to be a little more efficient and generate a little more boost. My recommendation is the ATS ported housing and matching wheel. This should raise your operational boost level to eliminate the backpressure issue and also provide an increase in performance as well. The other option is to use SES eliminator to turn the light off, but remember that it really isn't eliminating the problem... just the light.