1st gear problem in 68 cj5 trans

I have a 68cj5 with the 225 v6 and 3 speed. when I shift into first the trans works fine up to 5mph or so, then it starts to make loud clicking noises and eventually pops out of gear. I have no idea what this could be... any ideas? Also, what transmission do I have? Is it the T90?

 

if stock, would be T86, which was used with the V6.
pics of the trans would help
if the Jeep was a 4 banger, could be a converted T90 as well.
based on your description of the noise, I would pull the floorboards and the trans top cover for a peek; may have to drain/lower the trans oil level for full view.
metal chunks could fall out with the oil while draining.............
could also be motor mount(s), trans mount, exhaust hitting somewhere, lots of things to visually check as well.

 

It should be a t86 or t14, if the 1st gear is sychroed it is a t14, the t86 is very similar but is not syncronized in 1st gear.

 

Could it be something wrong with the overdrive unit?

 

mine was doing that till i looked and saw the shifter was hitting my cb and wouldn't go in gear all the way.