sluggish shifting

I have recently aquired a 1995 explorer auto w/4.0l from my father in law who passed away. The problem is...It doesnt want to shift and will "tach out" unless I "burp" the throttle at the appropriate rpms. He was having this problem so he had a garage service the trans. Still does it. Are there sensors that regulate shifting or what else can I look for....Please help.

try your shift linkage, i had a friend with a topaz that wouldnt shift until redline but when he took it in the shift linkage had slipped off, so they put it back on and it ran fine