Changed rear plugs and now transmission slips

It was a county school board vehicle so it is old but it was well maintained but then it sat for a number of years and I do not know if it was even started for years before the dealer I bought it from obtained it. It had 62,000 miles on it when I bought it a few months ago and I've put 10,000 on it without any issues until now.

A few weeks ago I started feeling a little vibration and the van had a rough idle so I changed the plugs. Instead of removing the wipers and cowling at bottom of windshield I jacked the vehicle up and accessed the rear plugs from underneath. Well she ran better than ever before but the next day the transmission started slipping. The engine would rev up but the transmission would not catch a gear unless I let off the gas. It is worse today and I've already put it back up on jack-stands and checked wiring to make sure I didn't knock something loose when I was changing the plugs. I have been out of work for a year and am supposed to start a new job Tuesday so I am very concerned and need help. Thank you.