kenny, the 99 doesn't have the vac disconnect, unless an inexperienced shop put one in as a replacement. (don't laugh, the 93 i bought had a vac disco installed at the tune of 700 bux, receipt in the glove box).

either that, or you don't have a front driveshaft...

 

Check your vacuum line they may have a crack or got pulled out. The same thing happened to me on a trail in my 88

 

i think i have the problem found is i checked all the line and they were all good checked the vacum coming from the switch on the tc and it is not changeing over to engage the front axle took it off and the bead on the end of it is sticking the new one will be here tomarow when i get it change will give you guys an up-date but sofar thanks for the help