I used to live right by I81 here in Maryland, and I drive I 95 down twice a year.
I'd recommend waiting until you hit Maryland to cross over, but you will notice that I81 goes southwest, taking you further away from 95. If you want to maintain interstate travel, here are a few of your options
I81 to I78 (just south of Pine Grove, PA exit 91 or so)--takes about 3 hours to get to NJ Turnpike, which merges into I95 in Delaware. We travel from Maryland to NY a couple times a year and take 81 to 78 to LI. Okay drive, but heading south on the tpke is pricey.
I81 to I70 (Hagerstown, Maryland, just south of exit 5)- This option takes about an hour and a half to get you on I95 proper, taking I270 south in Frederick to the Capital Beltway (495). Very familiar with this trip and I know this one like the back of my hand. The advantage is that you miss all the tolls of NJ, Delaware and Maryland.
I-81 to I66 (south of Winchester, VA) This one also brings you into the Capitol Beltway, about 3 exits from getting onto 95 south. Done it, it's okay, but if you try it during morning rush you won't be happy.
I81 to I64 (Harrisonburg, VA) You miss all the DC garbage this way, but you've already moved quite a bit away from I81. Brings you out at I95 in Richmond, VA. From Richmond, WDW is about a 13 hour drive.
Further south, I'm not much help, but if you want info about any of these routes, feel free to PM me...I'll be glad to give you estimates of distance, times of day that are bad, hotels from about Harrisburg Pa down to Hagerstown, etc. Depending on where in upstate NY you're coming from, you'll probably want to stop somewhere in PA that first night. If you're coming from Binghamton, Hagerstown is 5 hours on a bad day!
Also, if you aren't driving through the next day, here's a web site that I love
http://www.usastar.com/i95/lodging.htm
I've transported their info into a speadsheet, then whittled it down to places we'd stay/eat in each state...those pages are now stapled to the corresponding page in the road atlas. Makes it so easy when someone says "I'm hungry" and you can figure out what food is coming up that they'll eat!
Suzanne
all those years of being Dad's trip navigator growing up really have been useful!