When coming to a stop my 1999 GMC YUKON like shift downs kinda hard when i fully come to a stop?

Very common complaint, GM had a bulletin for this concern. it states

#01-04-17-004B: Clunk, Bump or Squawk when Vehicle Comes to Complete Stop or Accelerating from Complete Stop or Accelerating from Complete Stop (Replace Rear Drive Shaft Nickel-Plated Slip Yoke)
The cause is a slip condition between the transfer case output shaft and the rear driveshaft yoke.

Replace the rear drive shaft slip yoke with a new nickel-plated slip yoke. here is the part #

12479383 Yoke Asm.( Nickel Plated)

Must Be Ordered With U-Joint Kit P/N 12479126

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You may have a broken motor or transmission mount. I had a Lumina and when I slowed for every red light, there was a loud "clunk" when it downshifted into 1st gear at about 10mph. This also allows the vehicle to rock forward and back whenever you put it in PARK.

Check for the brake fluid level on your brake fluid reservoir (master cylinder), the condition of the brake pads, and any rust on the brake discs.

Change your automatic transmission fluid.