Dodge Repair: 87 Dakota low oil pressure and engine knock, low oil pressure, gauge reading

Question
My 87 Dodge Dakota 4x4, 3.9L, automatic trans has an unsolved problem.
 At cold operating temperature oil pressure (op)is 80 psi in park/neutral, drops to 40 psi in drive at idle speed, and op increases as engine rpm increases.
 At normal operating temperature op is 20 psi in park/neutral at idle speed. OP drops to 0 psi in drive at idle speed and lower engine knock begins. Engine knock stops and op increases to 20 psi, when trans is placed in park/neutral or engine rpm increases while driving.
 External op guage verified dash gauge reading was correct. Previous owner had rebuilt engine installed. Oil pan has been dropped, everthing looked ok, and new oil pump was installed. No change in op or engine knock condition.
 Could a broken crankshaft cause the problem?

Answer
due to the complexity of the problem I would have to say have the dealer diagnose the problem since low oil pressure leads to excessive wear and siezing of the engine have it done asap.. good luck with your problem and let me know how it pans out.. Joe