Reverse is not working?

What I mean by locked up is the button that you push to change gears does not go down, it seems it is locked up in nutural.
How do I chceck the shifter linkage?

I just bought my truck from a dealership about 3 months ago and the service light has come on and off a few times. I had to replace the alternator about a month after I got it and everything's been fine since. My car kinda vibrates when I go about 85 or 90 mph but I dont do that unless I'm on an interstate so I've ignored that. I was driving my truck yesterday to a friends house and it was completely fine on the way there - I put in park waiting for him to come out and when I went to put it into reverse - It wouldn't go. So I just turned around going off the road and then when I hit the gas - It made like a screaming sound - He said he thought it was a belt. We pulled over at a gas station and I got gas and he checked everything and couldn't see anything wrong. Well when we tried to drive it again - It now won't go over 35 mph and it is now stuck in 4 wheel drive. And won't go into reverse. I'm taking it in tomorrow but could it really be the whole transmission? The trucks in good shape and even the battery is only 5 months old. It has 103,000 miles on it but I didnt think that was enough to have to replace a whole transmission. I was never ever hard on my truck and when we bought it from the dealership - He printed out reports and stuff saying everything was new. My brother checked all the fuses - Nothing. Is there anything else it could be?

I have a 2000 blazer with 256,000 (mostly highway) 4X4 automatic with a 4.3 liter engine. Have had no signs of anything going wrong and yesterday my reverse went out. Have all other gears but no reverse. Please help with what the problem could be. Thank you.

Alright, just wanted to update you on what the problem really was. Didn't need a new tranny. The sungear for reverse went out, yes it was in the tranny. Had people guote $1100-1500 to fix it but I got it fixed for $500 that's plus tip, lol. Sungear was stripped and that's why I lost reverse with no signs of any problem. Mechanic said with 260,000 miles on car, the tranny looked awesome! Clean! Ok just wanted to update you guys on what happened in case anyone else has this problem. :D :D

Resently when backing up my truck, it does not want to back up. I press on the gas and it reeves up and eventually it does back up. Is it a transmission problem?

Thanks Joe. I will check it out. I know the fluid is good, I had the fluid and filter changed hoping that was the problem but it still didnt back up right and going foward is not a problem. I feel that I cant park on a hill because it is such a problem backing up. So I will definately have the linkage checked.

My 96 blazer acts like it is in neutral when in reverse, and when in drive I have to take my foot off the accelerator before it will shift. Any ideas?

Unfortunately there have been other issues with this vehichle and the check engine light is almost always on. Would this cause the no reverse situation? I don't know if it makes a difference but every once in awhile when I would back up there was a bang and a jolt

No reverse.

Yes all forward gears work.