Start a new question...in my 99 silverado if I put it in drive it wont move but if i use 1st then 2nd then drive it works fine

that miens 3rd is gone

Does it have enough transmission fluid in it? Have it checked first. If it is full, and clean and clear, I'd really think of having it pulled and checked. The bands or pump may be done for. Make sure the linkage is working correctly.

I would try a cheaper way and do it myself drop the pan carefully and then changed filter and fluid . while pln is down and old fluid hasnt been desposed of yet , get some between thumb n fingure rub together . if its just gritty new filter n fluid works ,sml. metal shavings , yes its time to get a professional a go at it . i do know a few other cheaper ways to go . thank you Mark Pope wf , tx.

Did you ever fix problem my is doing same thing after the rearend got changed