umm this might be silly but if you can put that model into 2wd then see if it slips would it help narrow down the problem area? or maybe if you put the whole truck up on blocks and run it (yea i know sounds crazy) if it doesnt shake then you can rule out several possibilities (driveshaft, slip yolk)and highlight others (anything around the wheels) and if you it does shake than the opposite is true...

or is that just crazy?

 

Is the truck stock or lifted? I guess you'veputthe truck up on stands and run it in gear, and looked things over?Got any runout? Also,just for grins,check the bell housing bolts, trans bolts, and the bellhousing for cracks. Look all mnts, bolts etc. over real close. Sounds like an angularity problem, but thesewill notsuddenly change unless something's loose/broke/bent.Unloading would cause the pinion angle to change slightly downward.Staying on the gas would keep the pinion up.It's the only thing I could see that would change, by loading and unloading, if everything else is new,tight,and in line,and no runout. It is a strange one.

 

PROBELM SOLVED, if anyone ever needs to know, the driveshaft was about 2 inches too short, casuing the slip yoke to pull back away from the splines about 1/2". This was giving the shaft too much play,youcould not move it by hand, but while driving it was throwing the angle off. Put a longer shaft in, "poof", no more vibration!!!!! Thanks for all the help and advice.

 

howd you fiugreit out?

 

I was thinking the driveshaft might have been warped or out of balance, the guy I bought the truck from had a few laying around, so he gave me one. I put it in the truck and no vibration. I pulled it back out and put the old one in, thats when I noticed the difference in length.