This is a condition where the clutch discs in the posi stick/slip. It is very common, and a function of preload, wear, lubricant, etc. There are a few solutions: replace the fluid with synthetic (Jegs has it, of any auto store now a days). Or add the GM additive if available. Worn or damaged posi's will not be fixed by ANY fluid change - time to spend $450 + setup charges.
