I had my mechanic check the diff oil today, and the level was fine, but he said that it was very thick, like 90+ weight gear oil.

We thought that a posi unit would have had a lighter oil in it.

Are we wrong?

GM makes a small bottle of additive for all the older positraction rears. I used Pennzoil 70-80 gear oil and two bottles of additive. If in  tight circle or figure eights, the rear chatters or clunks, you need to drain it, put new oil and additive in. THEN drive in tight figure eights for a few minutes, going both directions, it should seat the clutches and quiet the movement.

I think some of the newer synthetic diff oil already has the additive in it.

Well, there is no clicking noise, although after the last install of the engine I hear a whining noise somewhere in the drive-line.

I didn't know if I would be better just leaving it as is, or have the gear oil changed.  Sometimes changing things can create problems afterwards . . .

I don't remember for sure but I believe I put 80W in my differential plus the Positraction additive from GM.