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Ok the old work beast came with a 4sp w/overdrive that has since been replaced by an old granny 4 (cast iron borg I believe). It has been a looong time since I cracked a 4/sp case open. The problem is this while driving down the road at slow to moderate speed (up to 45 or so) it will slip and catch with a slight lurch (kind of like a miss) but it is in the tranny. when in 3rd or fourth gear and I let off the gas it will pop out of gear and mildly grind. I push in the clutch and let it out and it is back in gear. My gut is saying the shifting fork is worn down enough to let it out of gear and possible syncro wear..
The shift forks are easy enough to inspect by pulling the top cover. The forks typically have nylon pads that are easily replaced.
When removing the cover, you first have to remove the shift lever. There's a little pin on the side that can easily fall into the transmission. Details on how to safely prevent that are a couple of paragraphs down here:
There are a couple of other possibilities for a transmission jumping out of gear.
Make sure the bolts are tight on the front and back of the bellhousing. Any looseness between the transmission and engine block can cause erratic behavior.
If those bolts are tight, and the shift forks are good, axial play in the input and output shaft bearings is your next likely culprit. If the input and output shafts can move away from each other, that will cause jumping out of gear, usually on deceleration.