Motor mounts or transmission?
 

I have a completely stock 98 Sport that is having some kind of issue but I can't figure out what the problem is.

When I go to take off from a dead start it's almost as if the transmission takes a second to catch up with the motor and I feel this "thud" when the vehicle takes off. The transmission does not slip and shifts out smooth. It almost feels as if the motor is lifting up on take off like it could possibly be motor mounts? I am just guessing. Would bad U-joints cause this problem?

I wish I could explain it better. If you need more info just ask I will try and describe it better with some help.

Most likely a motor mount.

open the hood and power brake it see what moves...

That's a good idea. I did not think about that. I will try that and see if can tell anything from that.

That's what I kind of suspect although not 100% sure. If it is, how hard is it to change out motor mounts?

it's pretty easy, use a floor jack & a block of wood to support the engine.

It wont hurt to check the u-joints either. Wiggle the drive shaft up, down, left, and right. If there is play, your u-joints are goin. Do the same with the front shaft. Mine are both solid and I have no problems.

You might want to check your slip yoke also. Mine was causing a big thud when I started from a dead stop. It's an easy fix, just needs to be lubricated. Do a search on here and I'm pretty sure there's a few instructional threads with pics.

Also get under and take a look at your transmission mount. Feel it with your fingers. If it is shot, it will feel somewhat spongy