Need a mechanic's advice

[Tennman]

On my 72 Chevy 3/4 ton 4x4 ,the front drive shaft turns all the time . I can jack up the rear of my truck and put my truck in gear and when the rear starts turning the front driveshaft does the same thing . I can grab the front drive shaft and stop it with my hand and with little tention . I know its not suppose to turn while going down the road , but , i know it does and this is not good . Tell me if i can axcess the rear of the 205 transfere case and shim this sumhow and stop the front driveshaft from turning all the time . I had another truck ,in 1982, do the same thing and i had a mechanic to fix it . Any you guys done this and might help explain how to fix my problem ? I hate to take the round cover off the back of my transfere case if i don't know what to do first . I know i can do it , just never have ! Thanks for any help here guys .