I'd start by wiggling wires with it running in P/N. Reason being is when you drop it in R vs D the drivetrain rocks in a different direction, might be as simple as a problem wire. Check connections at the pcm (ensure the two bolts are snug). Check the connection at the ICM, that should also be snug.
If you think fuel pressure is a concern, test the pressure before shotgunning a pump.
CAT, if you think cat is an issue, rev the engine to 2K in P/N and hold it. See if the engine begins to labor like it has a plugged exhaust etc. Look at the cat after holding it at 2K for a while..is it glowing or excessively hot? Are the exhaust manifolds glowing?
Start there... that should give you a few minutes of stuff to try.
