It's been like this since I bought the rig, but it's annoying.. when off the throttle, most noticably in high gear, the tranny makes a noise like there is excessive play between the gears.. Under throttle, nothing...

It very well could be worn out, as the donor had unknown milage, but under load, it's fine, shifts fine (well, except into low), and there are no unusual looking pieces in the oil.

Should I be worried?

Also, I'm guessing the shift fork is slightly bent - going into 3rd & first (the "down" gears), it just doesn't feel like it's getting the best of engagement... I'm against the stop to get it to engage... and it's false-engaged 3rd a few times... is that the shift fork, or an adjustment?

for the noise when you back off the gas I would bet its the internally splined spud shaft between the trans and tcase. They wear alot and get sloppy. When you back off the gas you are riding on the coast side of the spud shaft so it makes noise.

You can still get new ones and you would be amazed at how much slop will be gone with a new one.

As for the shifter thing.... no idea!!!! :(

new shift forks required, they have a stupid non replaceable coating that wears off.
if the coupler is that worn so is the mainshaft, I have 2 like that
new coupler is a cheap 1/2 fix, thats what I did until I find a couple of good mainshafts

Do you have a CV rear driveshaft?

-troy