I'm having (what I believe) to be major slipping issues in the TC on my truck. I planned on doing the Goerend TD and VB upgrade, but considering it's age...

The vehicle has 100k miles and I'm wondering if I should go ahead with a full tranny rebuild at this point to extend the life of my transmission, or would a simple upgrade to get it back to normal working status keep 'er going for a while?

 

If your TC is slipping a lot, all those little pieces are going through the rest of the tranny as the fluid circulates. Given the slippage and the mileage, I would rebuild the whole thing. If you're within driving distance of Dave, the great thing about him is that he won't try to sell you a complete tranny at a set price. He'll pull yours apart and see what it needs and rebuild it at a fair price.