shift lever problems

Hi
has anyone experienced this? I have a 95 Yukon 6.5 diesel, one day I start it up grab the shift lever to put it in gear and the lever clicks like normal out of park, through reverse, then neutral and into overdrive but all the lower gear will not click in and the indicator shows overdrive only. the shift lever moves but it just slops around like somethings broke. I crawled under the truck and I can manually engage the lower gears and it clicks in like it should. I'm thinking it's a shift cable, but why would it only engage a few of the gears if the thing is broke, I would think none of the gear would engage if it's broke.
Any suggestions or comments appreciated.