So... What did you finally find this problem to be? I have a 94 FWD with a whine that increases in both pitch and volume only under load of acceleration . As soon as I take my foot off the gas or coast at any speed, the whine stops. I have already done the $1 o-ring replacement for the power steering pump w/ no change.
While I was at it, changed the internal tranny filter, added and external tranny filter, and changed differential fluid. Differential had a noticable amount of fine metallic flecks stuck on the magnetic plug.
I'm hoping this might be a power steering issue as opposed to the differential. There's an order of magnitude difference in cost.
-Dan