We used the lynx buses several times on our last trip

.
We used them to go from : -
DTD to Florida Mall
DTD to I Drive
DTD just to the Crossroad shopping centre
We found them to be quite reliable

and purchased a 7 day ticket which worked out very reasonable. Their website -
www.golynx.com - has the different routes and timetables plus a trip planner

.
From DTD to Seaworld its bus number 50.
From Seaworld to I Drive its bus number 8.
From I Drive to Florida Mall its bus number 42.
check the website for the bus to Universal....looks like number 43