My LT155 is almost 15 years old and has been used very much over that time. The splined pulley that goes on the input shaft of the hydrostatic transmission is worn allowing the pulley to wobble and slip enough that the mower would not move. After removing the transmission I found the input shaft splines are gone as well. The two spacers, one on top and the other on the bottom of the pulley still have enough of the spline left to fit snugly on the input shaft which also in those two areas has some spline left.
Carried it to a local JD dealer and they said JD didn't sell the shaft separate and I'd have to replace the transmission, a $969 plus tax deal. I said "no thanks" and left. Driving back home I went by a local welder to see if it could be tack welded and he said "No". I still think it could be but didn't have access to a welder myself. I started looking on Craigslist and found several mowers claimed to be in good to excellent condition for sale between $400 and $600.
I went by an auto parts store and bought some JB weld. Aligned the pulley on the spacer and placed them on the input shaft, JB welded everything together and let it set for 36 hours. I installed it back on the mower today and mowed almost an acre with it, using it in forward and reverse as I normally would.
So far so good.