I had the same problem on my 99 this past summer. Dealer said it was the solonoid. Cost $85 for part and $70 for labor. Problem unresolved. Dealer could not explain, and since car is out of warranty, I refused to pay any more. The problem was, the could not prove to me it was the solonoid, and did not have any troubleshooting methodology to support theory. So I took apart the console my self and watched how the "lock engages". It turns out there is a switch right above your brake pedal that opens when you step on the brake. I squirted this with a little electrical cleaner and this fixed the problem. My suspicion is that the hot and humid weather was causing this to stick, and this fall/winter, I have had no re-occurences of shift lock "locking".
-WSC