Does your transmission have any leaks that you have been ignoring? The trans on my son's 94 Ranger gave up the ghost one day when he was headed home from college for the weekend. I had known it was leaking for some time and just ignored it. I bought a used trans from a salvage yard from a 97 Ranger for $600. Spent almost the entire weekend on my back in the garage. I have never had so much trouble pulling a standard trans in my life.
It sounds as if your trans is low on juice and it's on it's way out.