Slipping Transmission

[lsupcar]

As the original owner of my 1992 Pearlie, I hate to think that it might be dead and what to do with it.
For several months I have had issues with clunking from the driveline. I initially thought it was the driveshaft after changing the fluid and filters with no change, but on further investigation, the tranny mount was split and after replacement most of the clunking went away.
However, the driveline started binding, especially in reverse when making a sharp turn, as in when leaving a parking space. Took it to a transmission shop that replaced the tranny at 100k miles and 15 years ago. They could not really replicate the experience, as it was intermittent at that time.
With time it got worse and thinking it was the transfer case clutch pack or duty solenoid c malfunctioning, I thought I would give the tranny additive a try as discussed in these forums. I put in two oz. of Lubegard tranny shudder fixx on Saturday. Drove it then, and it seemed to have helped. Didn't drive it Sunday.
On the drive into work this morning, all was well until while waiting for a light to change so I could turn left, I was unable to accelerate away from a stop. I could move in reverse and when shifting all the way to 1st. Once underway, was able to shift to D and made it the next mile to work.
At lunch, I drove the 2 miles to the transmission shop and dropped it off for them to look at it. Driving there, it again had the same slipping symptoms in D. When in that position, the car could rev to 5000rpm and not move at all. When shifted to 1st, it would thunk into gear and then drive ok.
I am hoping another fluid change might reverse any problem I may have caused with the additive. Or maybe there is another unrelated problem.
If it needs a new tranny, I am not sure I am up for that.
But then what to do with the car?