trans problem ??? need help

I have a 93 GMC Sierra, 350, 5 speed M48 trans. I know they put this in the 1/2 and 3/4 ton trucks. I put a new clutch in it about 3 months ago. Never really drove it all that far. This past week I drove it an hour away, had to go down a pretty pot holed muddy road for about a mile. THen the same in reverse to come home. The next day I found the truck to be shaking a shimmying on well speeding up or letting the truck itself slow down on down shifts. Both front and rear U joints were bad, changed them and it drove like a dream,, for about 24 hours and started all over again. Double checked the u joints and they are fine. It starts at the shake and shimmy at 30 mph untill 40mph, then the faster you go you can feel a harsh vibration in the shifter and clutch.. I dont know what it is???  

the mud could have gotten to the clutch assembly which is throwing it out of balance or the clutch you put in could be defective. that is just a guess, I would check the area and see if there is something stuck up in there are signs something got damaged.  

Sounds like what happened is the trans heated up from the drive and when u hit the mudd puddles the heat mixed with water caused the clutch disc and or pressure plate to warp. If you bought it from a parts store and is less than a couple months old, try to get them to defect it out. Chances are you can find someone either nice enough, or stupid enough to do it. I manage an advance auto parts and even I would take care of it.