Trans clunk only in drive but not overdrive??

I have a 1992 K1500 Blazer, love it, but this is kind of wierd. If I drive it in drive, when i come to a full stop I get a nice noticeable bang as it downshifts from what I believ to be 2nd to 1st at about 5-6mph as im almost stoped. This does not occur when its in over drive.. what the F? Now when i bought it, it had a clunk from what i thought was the rear, someone told me to pull the drive shaft and grease up where the yoke goes over the transfer case splines, so I did, changed the seal while i was at it, it was good for about a month and then all the grease dried up and the clunk came back, so i did it again and but this time I used anti-sieze and its been good for about 6 months now (basicaly im too cheap to buy that $250 nickel yoke GM says is the solution to that clunk) so I have ruled out the yoke being teh problem, its definatly the trans. Im not even 100% sure which trans is in this truck, my 86 K20 had a 400 which was cool aside from not having OD with 4.11's, that was not cool. Is it a 700 R4? I dont even know... The truck has 145,000 on it and I dont know the maintance history. The ATF looks full, clean and it smells fine, but im not going to change it anyway being its so old.

So what is sticking in my trans which is making that clunk as it shifts into 1st coming to a stop, what check valve is in a different position in my valve body when its in OD? Is there an additive anyone trusts I try before I look into this further, im not a big fan of all those gimmic products people put in their civics with 200,000 miles on it to get it to stop smoking before they sell it, i would try something some on this site reccomends from personal experiance.

Thanks