o/d off

the trans drives fine its just when you down a hill and take your foot off the thottle the truck free wheels like its it neutrual

hi i have got a 1995 nissan mistral that is free wheeling in drive,the trans has been replaced recently with a second hand one,the old trans had the same fault as well as others,im thinking that it may be electrical,i have changed the thottle position sensor but it didnt help.has anyone got any ideas?