Assuming that the OP's has a T150, it would use the same bellhousing as a T176 four speed. Everything else would have to change and the rear driveshaft would be very short. IIRC, John Strenk swapped his T150 for a T176 and documented it.
I purchased it without a tranny and was told by local shops that the three speeds are no good. They also said that I would need to modify a few things. I have the original I6 engine.
My '79 still has the original 3 speed. That's 33 years and counting with a tranny that's "no good"...and it's taken a lot of abuse over the years. It would be nice to have another gear on the highway I guess, but in the offroad I always liked having R and 1st lined up.
Yeah, the original T-150 is a very tough tranny, it's only shortcoming is it lacks one more gear wich would make it better for offroading because the 1st gear can have a better ratio. But for 90% of the driving you might run into, it works just fine. I ran mine for decades before I swapped. The T-150 is also very short just right for the CJ5
I changed because I went with 33" tires and was to cheap to swap gears in the axles. The little boost in 1st with the T-177 is great along with the better ratio in the Dana 300 t-case.
Not saying I wouldn't switch to a 4 speed if I ever have to replace mine. But I wouldn't rule out a t150 just because someone says they're no good. If you're on a budget and have a huge project you could probably stick one in for next to nothing, free up a little time and cash for other areas, and never have to worry about it again. But if you really want a 4 or 5 speed I guess there's no better time than now.
Yup original t-150 in mine. For a couple years I ran it with syncro rings that had no teeth left. Basically if you did not shift it just right it would grind and every tranny oil change came out real glittery. I finally decided to rebuild it and couldn't believe that the actual gears were still perfect. Changed syncro rings and threw on a new gasket and have been running grind free for the past 10 years. I don't drive it hard and put it up wet but I don't baby it either. Would be nice to have a lower first but I don't want to have to change other things out to make that happen. Plus when I bought my 258 the guy threw in a spare t-150 for $75.
Like others have posted, the T-150 is no slouch. My '77 has one behind the 6 and I explore with it comfortably and have been for years. I have 31" tires a 3.54 rear end and what I do miss is a 4th. But even with my three, I approach 65mph and can hold it there. Granted I'm an old man, but I'm not so sure I'd be comfortable going faster in a narrow track Jeep.
So if all that's holding you back from rolling is a transmission and one becomes available for a great price, consider it.
I'd rather be lost on the trails than found at home!