hi i have a 1990 toyota p/u and my 4th gear would work sometimes but eventually pop out of gear and grind like a siv and just wondering what could cause just that gear not to work and all the others work still been goin on fer about 2 years just want to figure it out had a similar problem with a 84 nissan truck but the tran ate itself pretty good just wondering if somone could help if u ca it would be much appreciated thank ya

 

It sounds like the synchronizers for that gear are shot.
Its usually better/cheaper to find a used tranny...it'll be tuff to find a good manual trans repairman