I'm working on the same problem with my 86 Bronco II 4WD. Months ago it would upshift OK but on downshifting to a stop it felt like it stayed in D too long so you would get a clunk/slam. I chased that noise for weeks replacing bushings,mounts,joints,etc and then it just went away and the tranny shifted perfectly until now. It was rebuilt last year (20,000 miles) and fluid is full and clean. I've replaced the VSS at tranny and have a new vacuum modulator and filter/gasket ready to install; just haven't got around to it. I'm leaning toward maybe a stuck governor but I'm not looking forward to removing the Tcase.