If I were a betting man, which I am not, I would put money on the charging issue. It's easy to check, disconnect the sediment sensor wire, make sure it's not grounded and if the light stays on with the engine running your alternator is not charging, or charging enough. It's a simpler fix than you think. I had this many times before, when my brushes were worn out. I replaced the brushes and it ran for a few more years. You can do it right on the truck no need to remove the alternator if you don't want to. Just pull out the brush holder and solder in new brushes and push it back in. It's a bit tricky because the brushes are spring loaded but nothing too crazy. Any overhaul shop can do it in no time if you have one in your area. The other possibility could be the voltage regulator that is faulty, if you find the exact toyota part it's an easy swap if not any old 24 volt external voltage regualtor will work it's just the proper wiring that will be a challenge.
I was done with the drama of my old alternator and installed a universal 40 amp one I bought from Australia on ebay. You have some adjusting to do to get it to line up and replace the pulley with a smaller one but it wasn't too hard to get it sorted. Radd cruisers in Duncan sells them as well.