4Runner is a 1985 22re with a W56 transmission. I assume the transmission is original, and has probably been rebuilt, but I don't know? The motor is not original and the previous owner added a second transfer case, so I'd be surprised if nothing has been done to the transmission. The odometer is over 240k.

Trans and xfer cases shift very nicely, and all have basically fresh Redline MT90. This is primarily a trail rig, so I haven't had it out on the highway/freeways hardly at all. Anyway, 5th gear had a noticeable whine that I wasn't really worried about two months ago, but driving it the other day it was LOUD. Way louder than before. The only diagnostic I've had time to do is check the fluid level, and it's fine. It whines only in 5th, and only when giving it gas to accelerate or maintain on an uphill.

Multi part question: What is my likely issue? Is this something that will survive the summer so I can fix it properly in the off-season, or do I need to fix it ASAP? (I'm considering yanking it and sending it to Marlin, but to do that I'd be out of commission for a couple months...)