Can anybody please tell me what these codes are and how to go about fixing them (in a general way). When by Suburban is under fairly heavy throttle, esp when the rpm gets up, it sounds like the motor or something is kicking in and out and it blows plumes of black smoke when it skips.

Look in the sticky for FAQ's for OBD-II codes. (Sorry beat ya to it Turbine Doc)

236 - Turbocharger system
238 - Turbobooster sensor high voltage

236 is a common one, *usually* it's the wastegate silinoid. Look for around 17" vac at the wastegate actuator at idle. No gauge? Check to make sure the actuator rod is sucked into the vacuum actuator. If it is good, but check to see if it's hard to move in and out. It should be solid. If not, weak vacuum or leak somewhere.

I don't know if 238 is a result of a short somewhere or an indicator of an overboost. Chances may be that both are related.

238 could be an ungrounded circuit. Test the "B" wire from the MAP. It should read ~1.50v key on engine off.