zetor 6745 transmission problem ?

Hello. I have a 1976 R regd 6745 4wd. When in 2wd there is a heavy clunk that shakes the whole tractor. This does not happen in 4wd. My thoughts are that it is not fully disengaging 4wd and occasionally tries to rengage. Am i thinking along the right lines and where is the best place to look for the problem. I think model no is correct but its horrendous outside and reluctant to go and check until later.
thanks

i think you have to adjust the 4wd lever. the guide slot is mounted with bolts in slotholes so you can adjust it.