Sounds to me like the pump is no longer working. Unfortunately, unless you have access to a pressure gauge for the trans, don't know how to test. Another possibility I guess is a failed torque converter, but usually they will make noises and other warnings.
A friend of mine had a similar thing happen with his Exploder a few blocks from my home a few years ago. He was driving fine, went through a dip in an intersection and when he started to accelerate the engine revved and the truck slowed to a stop. Was like he only had park and neutral. Towed him to my house and we pulled the trans and found the pump had broken. Put a replacement pump in and was good for a few months until he got in a wreck and totaled the truck.
The problems with the coolers in the early 90s was not leaking, but clogging. The design used very small holes in the tubing for the fluid to flow through and those holes tended to clog restricting flow. That in turn tended to reduce flow to the planetary sets. That lead to more wear and debris that would clog the cooler more. It was a nasty feedback loop that wound up destroying the trans. Replacement radiators solved that problem as did bypassing and using a large external cooler. Regular fluid changes helped as well.