I doubt its a trans pressure problem. If it was you'd likely here it all the time regardless of what range the xfer case is in. But to trouble shoot properly start simple. Inspect the fluids in both for debris. I haven't read this entire thread so forgive any redundancy. To eliminate a "pressure leak" in the trans have the pressures read when its doing the howling. If the pressures read within spec then its likely a bearing surface in the xfer case. The trans in-puts into the xfer case, so if you are only hearing it in 4 high you've already narrowed it to the x case. You have more or less split the dictionary here between the trans and xfer case. Id say the high cluster in the xfer case has a bad bearing or thrust surface. Another dictionary split is... are the wheels moving when its growling? If so its xfer case if not then that points back to the trans and thats a whole nuther path, less likely though. My moneys on the hi cluster in the x case.
Sounds like its time to split open the xfer case to look at the bearings, syncronizers, clutch rings, hubs, gears etc...

Thanks for the reply but the trans does it in 2wd now so I'm sure it's the transmission