My '92 Ranger has the same tranny and had the same problem. It was fixed by simply changing the vacuum modulator. The part is on the outside of the case. It costs about $25. Since mine was due for a trans service (fluid/filter) anyway, I took the truck to a trans shop (DO NOT have the local Jiffy-lube place do this repair!) and they did it all for $125 including the new part. Problem solved. They said it is a common problem on the older transmissions. Hope this helps.