I recently bought a 91 ford explorer 4x4. I have a few things to find out on it, since it my first explorer. The kid I bought it front didn't take care of it and it has a few issues. All low fluids. It grinds into and out of 3 and 4 gear. 1,2, 5 and reverse are fine. No noise or grinding. I went to chance the fluid and this is what is found.
See pics below!

It also had about a liter of fluid in it too. What should I look for to replace? How long do you think it will last before it gos?

Also the 4x4 lights is dim on it and when I push it it turns on brighter but never turns off like the low button does. Never tries to use the low and not sure it if works. Not sure if anyone else has this issue or not? With the dim light and does it mean anything? Also it has automatic locking hubs. Wished it has manual ones.

Everything is stock on it as well.

Any help, or advice would be helpful, thanks!