'U' joint or transmission problem
 

Could it be that when the u joint is worned out, you feel it more at the of the 1st and 2ng gear **** (automatic tranny) than any other shift. It feels like someone is banging with a hammer in the back of my Blazer ?
Danyboy

In my experience, U-joint trouble has produced a high pitched squeal in reverse and first gear, but it might show as you described due perhaps to higher torque being applied during 1-2 shift than in other gears.

U joint
 

Thanks, for the help.
Danyboy

Depending on how worn the joint is you could hear anything from the squeal mentioned above to a light ticking when engaging the transmission to an all out "clanging".

One way to check is to hold the brake hard and shift from Nuetral to reverse then back to nuetral then to Drive. Most times you will be able to hear it.

Could also check for play but those things are notorious for getting themselves in such a bind when they are bad that you can't always feel it.

Or, after a ride check and see if they are hot. The factory U-joints cannot be greased so if they're hot they're out of grease and will need to be replaced.