1991 Toyota PU - lurching only while in gear

I have a manual 1991 V6 4wd Toyota pickup that ran incredibly well until recently. One morning when I started up it stalled on me when I put it in gear. This happened a second time. The third time I gave it a lot of gas and it was able to get it going. Since then, when I’m in gear (particularly low gear) it kind of lurches and feels like it’s getting intermittent gas. However, in neutral when I give it gas there aren’t any noticeable issues. I’ve consulted a couple of folks more knowledgeable than me about it and one guess is that the gears aren’t engaging well. Does anyone have different thoughts or agreements with this theory?