It just came to me what the problem is likely to be.
When installing the pinion in the housing the rear bearing race needs to be driven forward into the housing, or else it drags on the gear. Chev shows a special tool you put between the gear and bearing while driving the gear into the housing
(here is a pic of the chev took) . With the 4.11 gears I have been able to go between the teeth with a punch and seat the race. There needs to be clearance between the gear and bearing when done.