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Check your front drive shaft for starters. Make sure all the u-joints are good and also the splines on the slip joint.How much lift did you put on the truck? You may need a longer drive shaft. In either case you have changed the angle that the shaft is accustomed to and it may be showing it's age.
my 84 does this, i havnt really looked at it but it almost sound like the t-case is hitting the floor. could be a bad motor or transmission mount