clunking under the hood

i have a 92 cherokee and there is a "thump, thump, thump" noise while under acceleration and only while making left turns. it also make one thump or clunk at startup. i've checked the knuckle joints on the front axle, the driveshaft u-joints the motor mounts appear ok, i've power braked it in drive and reverse and the engine is staying put. it is really pronounced while pulling a boat or trailer .it's been this way for and has yet to get worse .

The drive shaft slip yoke might be binding. Have you tried greasing it?

U-Joints even if they feel tight can be bad, also the knuckle joint should be checked. In order to check you may have to unbolt one end of shaft to check movement. any loose or tight u-joints will need to be replaced. They can also be greased using a pin injecter to get at grease fitting.