Does anybody have a ballpark figure for a 99 Silverado K1500 4x4 Z71 Transfer Case ? Unfortunately I am replacing both transmission and TC at 90k miles.

Call your local chevy dealer and ask. I priced one out for a 2000 jeep grand cherokee the other day that had no electronics only Full-Time 4wd and 4 low and it was around $1300. I would expect around the same for yours since the silverados are a little more common than that jeep.