Well I write this so often, I figure this is a good place to put it. Everyone wonders why their transmission shifts hard or not at all. Manual tranmission use what I refer to as a Slider between gears. You have one slider between 1st and 2nd, and one between 3rd and 4th. In my Jeep catalogs, these sliders seem to be referred to as Synchro hubs, or sleeves.
The actual synchro rings play a part in shifting, but actually don't do any of the shifting. The synchros have a friction material on them, and their jobs it just to slow things down inside the transmission when shifting.
This picture is from my trasnfer case, but its the same exact thing. You can see the brass synchro in this picture. The slider would be the ring outside of these things. The synchro slows the spinning of everything down, so the slider can engage the gear. The slider has pointed teeth on it, so if things aren't quite lined up, they will still slide past things.
These pictures will help demonstrate:
This example, is assuming both the slider side, and gear side have nice points to the teeth still:
This example is after both the slider and gear have wear on them and the teeth are missing, the pieces can't slide past each other, and won't shift:
The sliders is where most transmission shops go wrong. Most rebuilds get new bearings, and synchros, and may replace a gear or two if they are really chewed up. However this makes a transmission shift good for about 6-12 months, then its back to shifting rough. Mainly most places won't touch the sliders as they are relatively exspensive compared to other parts inside the transmission. Also keep in mind replacing say a chewed up 2nd gear, and leaving a chewed slider in there, the chewed up slider will quickly chew up your brand new 2nd gear. You can file fresh points back onto the teeth in a pinch, and it will help, but the best solution is to replace the whole slider, and both gears in question. Back when I did my first Mustang transmission for me to replace everything in the transmission, sliders, every gear, every shaft, every bearing and clip inside, it was roughly $750 total. However this would give me an entirely new transmission. The rebuild kit was $150 for reference.
Lots of people ask about reverse grinding too. Normal 1st-5th gears are cut at angle, much like a ring and pinion so they don't whine. Reverse is almost always a straight cut gear so it will have tendency to whine. Reverse is also not synchronized in many transmissions. Between these two things if anything is spinning in there, it will grind when trying to engage reverse. The solution is to just put the transmission in another gear first before going to reverse. I always put the transmission in 4th gear to bring everything to a halt, then go to reverse.
As for making a worn transmission shift smoother, the easiest thing to do is match RPMS. Once your good at it, you can actually shift without using the clutch. However I still recommend using the clutch, but, its good to know if your clutch ever goes out.
This chart is for an AX15 transmission, 4.10 gear ratio, and 35x12.50 tires, used just for example:
To create a similar table for your setup you can use this spreadsheet: www.cdmfabrication.com/jeepgears.xls
Anyways back to matching RPMs, notice 21 MPH in 1st gear is 3200 RPM, but it is 2000 RPM in 2nd. If you pull the shifter out of 1st at 3200 RPM in this example, and pull it into 2nd at exactly 2000 RPM, it would go in smoothly even without the clutch as everything would be spinning the same exact speed. Use the clutch, but using this method will help with downshifs and upshifts in worn transmissions. Same thing with downshifts, 35 MPH in 3rd gear is roughly 2000 RPM, when going to 2nd if you reved the motor to about 3250 RPM, the shift would go alot smoother, again if perfect could even be done without a clutch (Again use your clutch! No sense in grinding things attempting to master shifting without a clutch).
I've also had good luck using a bottle of Lucas Oil Stabilizer in my worn out AX15.