Gear grind noise
 

Newbie question here.

I have a 1992 runner V6 5 spd manual trans. When I put it in gear with minimum load the truck goes forward fine. Once I put it under load I get a noise from the transmission area that sounds like a syncro popping out and a loud gear grinding sound. I have replaced the clutch, flywheel, throwout bearing, and pilot bearing. Put the tranny back and still have same condition. Was told that this sounds like the rear diff. Replaced it and stil same. Next advice was the driveshat slip joint took it to a driveline shop got new U-joints and they inspected it and said slip splines are good. Decided to try it with it in 4WD and not put the rear shaft in. Same noise. Deduced it must be the tranny. Took it out again and took to the tranny shop and they did not find anything wrong with the tranny. I told them to look at the tranfer case and insure that there in nothing wrong in it. I am now at my wits end and don't have a clue as what the heck else I can change other than the front diff. Once I get the tranny back I am going to try and put the back shaft back in and try it again.

So the thing goes through all the gears fine with it out of the truck.
Put load on and get noise.
Doesn't have any grating noises just rolling along.

ARRRGGH!!!

If anyon has an idea please help me to figure this thing out.

It is just more $$$$ with every experiment.