99 ford escort jumps uot of 5th gear when you let

The synchromesh is worn on the 5th gear.
Hold it down or swap in a replacement gearbox.

best advice is dont go 40mph in 5th gear... if you are going up a hill at 40mph in 5th gear i have no doubt that it is jerking, your putting too much stress on the engine. shift up to 4th or 3rd you will be fine. 5th gear is an overdrive gear. 55+mph

the trans is worn out.

slave cylinder is starting to stick try having fluid flushed first but you may need a slve cylinder

Worn engine mounts may let the engine and transmission move too much, also the shift linkages may need adjustment.

HI STEVE, FLUID LEVEL IS CHECKED BY SPEEDO SENSOR,HAVE TO PULL IT OUT AND IT HAS LIKE A DIPSTICK ON IT,ITS BURIED BY RIGHT HALFSHAFT ON TOP OF TRANSMISSION. ALSO YOUR SHIFTER IS CONTROLLED BY LINKAGE,SEE IF IT CAME LOOSE ON TRANSMISSION AND IS CLUTCH GOOD? YOU MAY BE ABLE TO DISCONNECT LINKAGE AND MANUALLY TAKE OUT OF 5TH AND ISOLATE THE PROBLEM.

Check the operation/adjustment of the 5th/Rev gear gate and associated selection levers.
Check 5/R gear lever linkages are operating correctly.

gear linkage worn joints.

no. linkage most likely you need to get adjusted or
one of it is worn out

The gear linkages are not correctly aligned.
Is the gearbox mounting in exactly the same place as when you removed it?
Even a 1/4 inch will offset the gears.
hope this helps