U JOINT WORN OUT? - I have a 1977 Sierra Grande that is providing weekly projects since I acquired it. It is low mileage at 130,000 for a 35 year ...

I have a 1977 Sierra Grande that is providing weekly projects since I acquired it. It is low mileage at 130,000 for a 35 year old truck, but has spent periods of it's life inactive, and parked outdoors in the Texas climate -so it has cracked dash pad, faded paint, bad rubber and seals, etc. I just got the ac working with help from this forum, and today something new popped up!
As long as I am driving along accelerating with my foot on the gas pedal, all is ok - but when I stop accelerating, and start to coast - a loud whine starts to come from the drive train - but if I push on the gas pedal and accelerate - it stops. Only while coasting/slowing down is there noise. My 1st impression is that I have a bad u-joint? It has a 2 piece drive shaft, and I've already replaced everything except for the u-joint between the rear end and rear of the drive shaft.

It sounds like classic rear end gear noise. The only real fix is to either replace the rear end assembly or have new gears installed. If you can live with the noise it'll probably last a long time. You may want to take the cover off and have a look at it and at least change the fluid.(I'm assuming this is an automatic truck. If it's a standard shift, it could be transmission bearings. you can put it in the top gear and that noise should lessen or go away if it is a manual).