I have this issue where whenever I decelerate under load or into a turn at low speeds I get this shutter through the whole truck. This happens especially in 1st gear which sometimes makes parking a hassle because it will decelerate very jerkingly. (It likes to fall out of 1st gear alot if I don't hold it in as well) I'm not too transmission savvy, but that's the only conclusion I've come too. What could the culprit be? Clutch? Throwout bearing?

 

If it is jumping out of gear the first thing to check is the shifter bushing and seat if they wear it can cause that.

Also as the bearings in the transmission wear it causes things to move making it easier to jump out of gear.

Your other issue sounds like your engine rpm is to low for the gear your in .

It might be something else but this is the first thing that comes to mind

Do you stall the engine quite often as well??