If the shop installed new pinion bearings, you would hope they would have removed any metal particles from the drive. One issue that could be at fault is the lower gears while turning will act like an oil pump and move the oil up to the upper. The cavity that the oil flows through could be plugged. Or metal from that cavity could have worked it's way into the bearings. Important to look at the bearings and see if they just welded together or if there was previous metal fragment damage. Several years ago I started my motor in the spring and the lube was low. I did not think anything of having it low in the bottle. Ended up welding the bearings together inside of about 2-3 mins of idle time.
So my guess is too tight on the preload, or no oil to the pinion bearings, or metal fragments caused them to seize.
Let us know what they find..
Dick