When I spin my tires I hear a loud noise underneath really bad. Have checked reardiff, seems fine, maybe transfer case?

need more info. what kinda noise. you kicking up rocks into the unibody (remeber thats like sitting inside a kettle drum, loud)

thanks... its anytime the rear wheel spinout like in the rain on pavement so its not rocks it feels like a slight hesitation then the noise is lik someone smashes underneath with a big hammer it happens every time and even with just a slight spin i thought mabe the rearend so opend up the rear diff and it all looks good no dirt no metal bits gears look good all nice and clean. could it be poss.. its trying to slip into 4-w-d thanks for any info planning to change the transfer case this weekend

Check your U-Joints

Stop spinning your tire's than you wont here it.Check your u-bolt nuts re-torqe them,broken spring.Or are you just mashing the go fast pedal to be cool,and making it have wheel hop.

Took the transfer case out yesterday and opened up the rear cone shaped housing first and found that the spot where the bearing seat is on the housing was sheared allowing the shaft to slip back and forth slightly so it was trying to pop in and out of 4 wheel drive on its own. Put a new transfer case in without any trouble and that should solve the problem.