IMO I would upgrade your binos and put off the spotter upgrade for now. You can upgrade your binos to viper 15s used for $400 and put $200 to something else, or get a used pair of kaibabs. The 18s in combo with your diamondbacks would be a good combo. I upgraded my 15s to meopta but still have...
Where in AZ are you? You can get a good deal on a used spotter with that budget. Viper hd works just fine. I had the 65mm until I upgraded to the kowa and it was sufficient. I assume You are already using a tripod with 15s?
Those 264AO tripods are probably the best value out there, not great for backpacking but when your 6'2" nothing really is that doesn't cost an arm and a leg. Throw a benro S2 on there and that's a solid set up.
What's your budget? You might be able to pick something up used that's a little better. Also look at getting refurbs at aaoptics.com, they still come with the lifetime warranty. Also will you be using a tripod to glass off of?