93 diamante overdrive problem

hi there. i have a 93 mitsu diamante and it has overdrive issues. when i drive it with od off it drives fine but of course doesnt go into overdrive. when i select od on, it revs up like its in neutral. it doesnt seem to be slipping, its like neutral. i have read previous posts and believe my od clutch is bad. 236 dollars from dealer but easily replaced. problem is im 150 miles to closest mitsu dealer!!!! so i was hoping someone with mitsu tranny expertise could tell me how to check it or if buying the "end drive clutch assembly" will fix it? any other known issues i should check as well? i am a "shade tree mechanic" who fixes my own vehicles, but am unfamiliar with trannys and especially mitsu's as this is first ever owned. any help is appreciated!