A couple different viewpoints:
3rd Gen T4R as a 3rd extra vehicle.
If you can afford the new vehicle without selling your 3rd gen as a financial cushion, consider this. As an add-on with minimum legal insurance, that's hardly any extra per month. You'll have to register/inspect it once a year and maybe a couple oil changes if you stick with synthetic? Some gas stabilizer in the tank so it won't go back from sitting. That'll be maybe $200-$300 a year to have a ready to go backup driver that's very capable and that you know is maintained well and reliable?
Keeping the 3rd gen as your DD.
If you put the lift, wheels, and tires back down to stock, you could probably stay close to 22-24 mpgs, especially if most of your drive is highway. You'll still have 4wd when necessary, and stock, it's still pretty capable.
When these trucks aren't modded and you stay off the skinny pedal, they actually make pretty good MPG.
You are probably only at half life with this truck, and you know the maintenance is good. You can do most of the work yourself, unlike newer vehicles, which saves significant money.
Based on what people are saying here, you probably won't get more than $7.5k for your T4R, and I'm seeing on CL that a base 2wd Lexus or Acura is starting around $12k, and those aren't in the best condition. So that's $4.5k extra at the minimum that you'll need to shell out, plus getting all maintenance up to date. Timing belt, fluid changes, spark plugs, etc. So if you sell the 4Runner and buy the Lexus or Acura, you'll need to pay an extra $5k at the minimum.
How many gallons of gas could that buy you if you stuck with the 3rd Gen?
Also, don't forget, a newer vehicle (especially a luxury vehicle) will cost a good deal more to insure, so you might not see much of a difference monthly. You'll probably want better insurance, too, because you can't just say,
"**** it, I'll go to a junkyard for a new bumper", when you get rear ended.
