Does the fluid smell burnt?
The motor/tranny combo isn't the prime suspect.....
I'm thinking it's a ballooned (ruined, toasted, junk) torque converter.
Neither the shifter linkage or detent would cause what you're describing.
If it isn't the TC... you've got severely toasted clutches in the tranny.
A quick sniff of the fluid should indicate the problem.
(Dropping the tranny pan and finding toasty flakes of clutch material will indicate a tranny/clutch problem.
No flakes... I would be suspicious of the used converter.