Well your timing is great as I have been dealing with this for a couple years and just fixed it on my 80 last night.
Most of the 80's that I have worked on with this noise is either caused by bad rear control arms/not adjusted for a lift or front driveline vibrations. Most 80 owners let it slide and don't worry about it. You can search this condition by searching "Slack Throttle" as that brought up most of the issues for when I was looking. I have a Dobinsons 3.5" VT coils front and rear with adjustable rear upper and lower control arms. Front and rear drive shafts are new. Rear is a new OEM shaft as my splines had play from 300k and the front is a double carndan shaft. Even though you are only lifted 2" I have seen this issue with stock 80's.
To fix this I would start at looking at your rear control arms. Most times the bushings have a slight rip which will let the axle move all over making bad pinion angles or just causing vibrations under slack throttle conditions. This may not look bad to the eye, but I promise it's bad while your driving. Replace bushings at minimum but I would recommend you get adjustable arms top and bottom to re-align the rear pinion. This noise always echo's like it's coming from the front driveline but is usually caused by the rear driveline angles. Second, I would install a double Carndan front driveshaft and I recommend
@landtank as his shafts are super quality and normally in stock. Please don't buy Tatton on eBay like I did and regret saving some coin just to curse his crap quality. Tom Woods is a decent option, but uses the same junk u-joints as Tatton which fail frequently. Landtank's shaft is built with OEM joints making them last for many years.
Because I am lifted higher than you I had replaced all of these components and still had a minor growl (even though it had gotten much better...). I just installed a set of
@Delta VS front radius arms last night and now my slight growl at highway speed under light decel is finally GONE!!!!!!!!!!
To my knowledge not many have actually posted a fix to this, so use my experience in fixing this on multiple 80's to your advantage.