94 fs Blazer with strange grinding type noise

I have a 94 full size blazer with the 6.5 diesel. It has been a reliable truck for years and recently has developed a grinding noise on heavy acceleration. If I accel slowly the noise does not appear, if it is in 4wd no noise. If I am stationary and torque the engine in drive no noise whether in 4wd or 2wd. The u joints look good, no noise when letting off the gas, no clunking sounds like a u joint when bad. I'm puzzled. It is leaking trans fluid from the front seal but other than that all seems to be working. I have had drive shaft problems and u joint problems on other chevys but nothing I see looks like a driveshaft/u joint problem. I think whatever it is it is related to the rear shaft, axle, or transfer case back to the rear end since the noise goes away in 4wd. If anyone has had similar issues or can give me some other ideas I would greatly appreciate it.

I have a 94 full size blazer with the 6.5 diesel. It has been a reliable truck for years and recently has developed a grinding noise on heavy acceleration. If I accel slowly the noise does not appear, if it is in 4wd no noise. If I am stationary and torque the engine in drive no noise whether in 4wd or 2wd. The u joints look good, no noise when letting off the gas, no clunking sounds like a u joint when bad. I'm puzzled. It is leaking trans fluid from the front seal but other than that all seems to be working. I have had drive shaft problems and u joint problems on other chevys but nothing I see looks like a driveshaft/u joint problem. I think whatever it is it is related to the rear shaft, axle, or transfer case back to the rear end since the noise goes away in 4wd. If anyone has had similar issues or can give me some other ideas I would greatly appreciate it.

unless the noise can be heard this is diff to dertermine for sure. My guess would be the rear diif. I would remove the rear cover and inspect gears for proper wear/engagement excessive metal/debris. with both rear wheels off the ground rotate them and inspect rear wheel/drive shaft yoke bearings for wear.

Check and make sure the actuator valve for the 4x4 is not partially engaging. Disconnect the vaccum for the 4x4 and go for a drive, if that solves it, the 4x4 is partially engaging.