Right after I got my 4Runner (1990, V6, 5spd, 4.56 gears, 31" tires) I took it on a trip up to the hills. There is a climb on CA 108 near Twain Harte that I have never thought of as steep before, but my 4Runner was down to 3rd gear chugging along at 45mph if I was lucky. I really questioned my purchase at that point, fearing the little truck was going to self destruct every time I wanted to go play in the mountains. It may have been able to go a wee bit faster than 45mph, but I didn't want to push things. I did not have any issues with overheating.
Shortly after that trip I did a basic tune-up: spark plugs, ignition wires, distributor cap, rotor and I cleaned my K&N air filter. I made another trip up that hill in very similar weather conditions (mid 80s, air conditioner off for the climb). When the hill approached just east of Soulsbyville I down shifted into 4th gear and tried to pick up as much speed as I could for the grade. To my surprise, I never dropped below 55mph nor shifted out of 4th gear.
The tune-up helped I am sure, but technique did just as much. The 4Runner of that era is a good example of a momentum vehicle, that is to say they do not easily gain momentum so be sure not to lose any! The 3.0L 4Runner will never win a race or climb hills with authority, but there are simple things you can try in order to get its performance back to stock levels again. Before my tune-up I was getting about 14.5mpg, and afterwards I get anywhere from 16.5mpg to 17.2mpg.
For me the hardest thing has to be changing my driving style when I get into the 4Runner. My other "transportation" will pull the same hill in 6th gear at any speed I would like it to. There have been times I had a vehicle get in my lane, slowing me down to 40mph and then when it moved back into the slow lane I could easily accelerate to 120+ if I wanted to (which I have never wanted to - that's way too fast!). Oh, the joy of owning a sport bike.
