Was the sender replaced during the rebuild? And have you verified this low pressure with another guage? Maybe a manual guage? I have had many GM oil pressure senders fail over the years. Scares the crap out of you. Usually they read fine at startup and then would slowly drop as the engine warmed up. Giving a false low reading, sometimes zero pressure! Just something to check before the hassle of dropping the diff and pan. Not saying it couldn't be a pump, you never know.
1994 Chevrolet Blazer 6.5L 4X4
200,000 miles, OPS/LP relay, new glow plugs, F style intake-EGR delete, new crank pulley, Spectra sending unit with ACDelco tank sock, new fuel & brake lines front to back, rebuilt (by me) stock turbo, new stock Bosch injectors, Heath PMD relocation with Dtech PMD, Heath fuel pump, new timing chain & crank sensor, air intake snorkel removed, new CDR, resealed engine (except head gaskets), new oil pump, Feed the Beast mod