well sir that is the exact same thing mine did. i stopped driving before it started locking up the rear wheels. the counter shaft is not the stoutest of units. although they are not that expensive to replace. what i am having to do. you know all this though, so i babbling.
i looked into the six speed convertion, you dont read a whole lot, next to nothing, on here about them dying. other than the cost to put one in there. for all the small parts and the transmission it was ballparked at 3k from one of the bigger names. that was for a new transmission. that doesnt include the part where you have to shorten your rear driveline a foot and extend the front a foot. its a big sucker.
aside form that the six speeds as per here dont seem to fall apart or break apart like the 4500's which i dont have a lot of good things to say about.