The slow engagement of drive is more than likely the forward clutch seal is going bad. This is a common problem with the C6. The seal is cheap and easy to replace (once you get to it), the problem is you have to tear down the transmission to get to it.
The clunk could be excess backlash in the rear.

 

I hear a clunking noise when I pull up onto uneven surfaces. I brought up that noise at the dealer and I was told its the rear axles and its perfectly normal. That noise is not normal. Next time I go back to the dealer I will make sure they thoroughly check it out.

 

 

The clunking may be "U" joints. Make sure the tranny is full of fluid or it will slip.

 

 

1. Ok so fluid is ok..I agree it sounds like it is coming from underneath rear part of truck, so ck u-joints. But if it slow to go in gear sounds like rebuild time if the fluid and filter are fine. When is the last time you did a fluid and filter change? Have you checked the trans modulator? But with that clunk sound....can you confirm that the sound is coming internally from the trans?

"Check the vacuum modulator and the line that goes to it. Check the line for kinks or other problems, make sure it hasn't come off.
If the line checks out, take the line off where it connects to the modulator. If trans fluid leaks out, its bad. If no fluid, it still could be bad. They're simple to replace, disconnect the vacuum line, remove one bolt that holds the clamp in place and the modulator comes out. It has an o-ring seal.
Modulators are identified by the color of the stripe. Pretty much the "go to" one is the black stripe. If they don't show that one in the catalog, go with the green stripe. Avoid the pink stripe. That one is a dual diaphragm version that has two vacuum line going to it. If by chance you have one of these, replace it with one of the other two and use the vacuum line that goes to direct manifold vacuum, cap the other line.When you pull the modulator out, be careful. There is a pin that goes in the modulator. Sometimes it comes out with the modulator, sometimes it stays in the tranny. If it comes out, don't lose it.

Transmission install tip-8" long grade 8 bolts with the heads cut off to make guide studs. These make installing the transmission a whole lot easier, and since I was doing everything by hand, easier was always better. These bolts have been put in their own box and labeled for future use. Use a hack saw to cut flat tip screwdriver slots in the ends, use a grinder to round the ends a little after cutting off the heads.

Trans tip 2- When installing a flywheel and torquing the bolts, you need to prevent the crankshaft from turning. I used a piece of 1/2" aluminum U-channel which I cut to length and slipped between two of the ring gear teeth. I then wedged it against the bottom inside corner of the frame at about a 60 degree angle. The aluminum is softer than the iron teeth, so any damage will occur to it long before the teeth, and it will actually conform itself to the iron teeth if enough pressure is applied. The U-channel also fits perfectly over the outside edge of an FE flywheel, so it will not slip off!
To actually install the transmission itself, I slid it under the truck on a piece of cardboard, then used two pieces of 2" round hardwood stock laid across the transmission tunnel as "hands"...I tied two pieces of rope under the front and rear of the trans, then ran them over the stock, then back down to me. This way I could pull down and push up from underneath, while having the option of tying off the rope to the frame to hold the trans suspended while I re-positioned.

 

 

Be careful doing that, chalk block the tires and use the e brake.