If you have a little time on your hands, and easy access to the block drains, use about 2-3 tablespoons of TSP (trisodium phosphate) - the real stuff, mixed with about 1/4 cup of powdered Cascade dishwasher detergent and fully dissolve into a gallon of distilled water. After you drain all the coolant from the engine and radiator, refill with your mixture and top off with Distilled water only and run for 20-30 minutes with the heater on high. Drain, refill with distilled water and run with heater on high until hot, drain, repeat until you see clear water coming out of you engine/radiator. Works like a champ, you will be surprised what you see come out of there especially if it hasn't been cleaned before. I used clear tubing I got at Lowes attached to the nipple extensions on the block drains and radiator so I could see the drain out and to keep things clean/dry. Block drain on the driver's side of the 4.0 V6 is a real joy to access if you have big hands and arms! There are a couple of good how-to posts about this somewhere on here. Diesel forums use a version of this regularly when they clean, or have a head gasket/coolant leak issue.
Very effective, and much easier on your cooling system than acid based cleaners. I did this process on the Tacoma in my signature right after Thanksgiving and refilled with Toyota SLLC - Pink. I just wanted peace of mind for another 4-5 years, water pumps suck on these V6's I hear but mine is hanging in there. I think that generation of Tacoma used Toyota Red; if you can find the concentrate at a good price jump on it. If not, Zerex Asian Vehicle - Pink is a good product, virtually identical to Toyota OEM Pink. I got a great buy on a case of Toyota Pink 50/50, that's why I used that. 2.5 gallons or so for my truck, another 2.5 gallons for my son-in-law's 4Runner, and a little on the shelf for top-off.