1996 Suburban rear end problems

1996 suburban 137k miles with lift_drives fantastic out of the blue oil light comes on I put in 3 quarts now oil pressure is at just above 20 on gauge at an idle and between 40-60 while driving_guy who checked truck says he thinks rear seal leaking-my back end locks up VERY periodically when barely tapping brakes too almost sent my son through windshield it locked up so hard like I was slamming on brakes this happened first over last few months then the oil pressure? Woman who has unfortunately been taken advantage of at Auto Repair Shops want some info before I take in so it won't happen again. Can you help me?