no, the tcase is not vacuum acuated. i've seen that happen if your running 85-140 gear oil or something really thick, but its usually just a slight spin, not really power being put through. are you sure the front drive shaft has power going to it or is just a slight spin?
When you put it in 4wheel drive and all 4 wheels are on jack stands all 4 wheels spin in drive. But when it is in park and in 4wheel drive it will not let the rear spin but the front dose.
