What is the towing capacity of a 1990 Toyota V6 4x4 pickup?

3500 pounds but considering the age of your pickup that number probably doesn't hold merit. I have an 89 Toyota pickup v6 single cab, and also a 1992 toyota 4runner, v6. I usually pull my bass tracker pro 190 boat. Its roughly 19 feet long and weights around 1100-1200 pounds. Both of my toyotas can pull the boat but going up steep hills can be a challenge Depending on how long and steep the hill is, i generally need to downshift and drive slow as balls up the hill. I typically put my 4ways on and creep up the hill. (The hill would be like a mile long at a 45 degree incline). So to answer your question, i would say your bad boy could handle up to 3000 pounds but pulling things is not really an enjoyably process. A bigger truck like a Ford f150 is so much different and easier. Last but not least, stopping can be even worse than pulling. You need to be extra careful about stopping. You can burn through your brakes pretty easily.