4Runnner transmission problem

I own a 1990 Toyota 4Runner, I've own it for about a year now. When I first got the truck it worked great, but then little problems started happening. The check light came on but nothing seem to be wrong with the truck. I month after that the O/D light started flashing alittle, whenever it wanted to flash. I got really neverous and started to figure out what was wrong with the truck. I started with the check light,I had it scanned and figured out it was the oxygen sensor that was replaced. After the check light was fixed, the O/D light started flashing more daily and slowing my truck down in first gear,(like if I was in low gear or something). It would only happen if I stopped at a stop sign or at a light. I was told I might have a transmission problem. I wasn't sure when the last transmission fulid change was and I was afraid to change it, thinking it might make it worse. I had the truck check out by a few people but no one can give me a straight answer. One person placed the computer on the tranny but found no codes. All I want to know is what I'm dealing with, I know my fair share of this about cars and trucks. I want someone to be straight with me not try to play me, like some unexperienced person. I hope someone can help me with my situation! CAN ANYONE HELP ME???

Also since I owned the truck, when winter hit I tried to put the truck in H4 or L4 the truck would tremble then shut off. It still does it till today. CAN ANYONE HELP ME WITH THIS???