play in driveshaft... transfer case maybe???
 

Ok got new converter put in. Everything is fine except a loud as hell pop when I get on the gas and let off. The sound is coming from under the truck...u joints seem to be fine and the noise sounds like its coming from the back of the transfer case.4wd works fine with no grinding noises. I can hear it when I shift from Reverse to Drive. Its a 48re by the way. Any Ideas? no abnormal vibration.:ouch:

check for play in the pinion on the axle.

check your carrier bearing, thats where the noise was comng from on mine when it took a dump

Had kinda the same noise in my 3500 as well and changed carrier bearing and found a couple u-joints that were bad. i would start there first

Check everything they listed, and also verify that the transmission mount is tight on the crossmember.