trubenzer

I have a 94 K2500 4x4 6-bolt hub, (175,000 miles) and from the service code in the glove box (MG5) I believe it is the Getrag 290 5 speed manual transmission. About 200 miles ago, the transmission seemed to have a slight vibration at various speeds when in 5th gear. Now, it seems the light vibration has become significant and is more of a rattling or grinding noise. In addition, the noise also is present when in 3rd gear. 2nd and 4th gear seem to be fine. 1st is hard to tell because it is so slow. Does anyone have any suggestions or insight as to what the problem might be?