Will a U-Joint off a 84 automatic 240 GL fit on a 81 manual 240 dl?? HELP!!

It depends on the driveshafts. But why hassle with swapping when new ones are only $8.00 at FCP Groton? Maybe $16 at NAPA.

In any case, ther are 3 possible U-Joint sizes, as measured across the yoke. I'll just call them Small, Medium, and Large because the vendors don't agree on the numbers (or how to measure).

There may also be two different driveshafts in those years. If the front section diameter is 1.75", the three U-joints (front to rear) are S S M

If it's a 2" shaft, the joints are L L L — So by checking the front half of the shaft, you can tell which size joint to ask for. The "nominal" joint sizes are:
Small = 2 5/8"
Medium = 3"
Large = 3 3/16"
--
Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and still here)
New 144 from '67 to '78
Used '62 122 from '63 to '67