Welcome and nice truck! You're right that you won't need a diff drop for '99 tall coils, but they're only like $25 and take 10 minutes to install yourself if you decide to go with a bigger lift. You can fit 265/75/16s with no lift too (not sure if you thought you needed to lift it for those tires). If the timing belt/water pump was done ~90K, you have until 180K to replace them per manufacturer's recommendations. Many people have gone well over 100K and even 150K without replacing them. I'm not recommending you do that, but it's not imperative right now at 175K. One good thing is the 3.4 is a non-interference motor so if the timing belt were to break, the engine will just stop but no damage will occur to the valves or anything else.